BUG: Blending fails with 0 weight for passed data

[SamGriffithsMO]

When calling improver weighted-blending via the CLI, it is possible to pass multiple cubes that all have weights of 0 (as the graph has no knowledge of the blending weights). e.g. tempmax12h, for a 24 hour leadtime being passed engluk and ecukx only.

Currently this results in blending.weights.WeightsUtilities raising ValueError("Sum of weights must be > 0.0"). It is desirable for this to be a warning so that suites can continue to cycle.

Acceptance criteria:

1. No change when presented with a full data set
2. If there is no weight to any input, raise a warning and return None

[cpelley]

Do we know if plugins that typically run after the blend support being passed None (currently that is).

In future, it could be that a plugin determines whether the blend actually occurs at all, based not only on the presence of a new field to blend but perhaps also whether it has a non-zero weight to one or more of these fields:

---
title: blend
---
graph TD

subgraph pressure_at_mean_sea_level[pressure_at_mean_sea_level]
0(["0<br>operation: blend_trigger"])
click 0 callback "leadtime: [0, 3600, 7200, 10800, 14400, 18000, 21600, 25200, 28800, 32400, 36000, 39600, 43200, 46800, 50400, 54000, 57600, 61200, 64800, 68400, 72000, 75600, 79200, 82800, 86400, 90000, 93600, 97200, 100800, 104400, 108000, 111600, 115200, 118800, 122400, 126000, 129600, 133200, 136800, 140400, 144000, 147600, 151200, 154800, 158400, 162000, 165600, 169200, 172800, 176400, 180000, 183600, 187200, 190800, 194400, 198000, 201600, 205200, 208800, 212400, 216000, 219600, 223200, 226800, 230400, 234000, 237600, 241200, 244800, 248400, 252000, 255600, 259200, 262800, 266400, 270000, 273600, 277200, 280800, 284400, 288000, 291600, 295200, 298800, 302400, 306000, 309600, 313200, 316800, 320400, 324000, 327600, 331200, 334800, 338400, 342000, 345600, 349200, 352800, 356400, 360000, 363600, 367200, 370800, 374400, 378000, 381600, 385200, 388800, 392400, 396000, 399600, 403200, 406800, 410400, 414000, 417600, 421200, 424800, 428400, 432000, 435600, 439200, 442800, 446400, 450000, 453600]<br>cycle_offset: [None]"
style pressure_at_mean_sea_level_blend fill:#D4A76A
subgraph pressure_at_mean_sea_level_blend[blend]
1(["1<br>step: blendgrids"])
click 1 callback "leadtime: [0, 3600, 7200, 10800, 14400, 18000, 21600, 25200, 28800, 32400, 36000, 39600, 43200, 46800, 50400, 54000, 57600, 61200, 64800, 68400, 72000, 75600, 79200, 82800, 86400, 90000, 93600, 97200, 100800, 104400, 108000, 111600, 115200, 118800, 122400, 126000, 129600, 133200, 136800, 140400, 144000, 147600, 151200, 154800, 158400, 162000, 165600, 169200, 172800, 176400, 180000, 183600, 187200, 190800, 194400, 198000, 201600, 205200, 208800, 212400, 216000, 219600, 223200, 226800, 230400, 234000, 237600, 241200, 244800, 248400, 252000, 255600, 259200, 262800, 266400, 270000, 273600, 277200, 280800, 284400, 288000, 291600, 295200, 298800, 302400, 306000, 309600, 313200, 316800, 320400, 324000, 327600, 331200, 334800, 338400, 342000, 345600, 349200, 352800, 356400, 360000, 363600, 367200, 370800, 374400, 378000, 381600, 385200, 388800, 392400, 396000, 399600, 403200, 406800, 410400, 414000, 417600, 421200, 424800, 428400, 432000, 435600, 439200, 442800, 446400, 450000, 453600]<br>cycle_offset: [None]"
2(["2<br>operation: save"])
click 2 callback "leadtime: [0, 3600, 7200, 10800, 14400, 18000, 21600, 25200, 28800, 32400, 36000, 39600, 43200, 46800, 50400, 54000, 57600, 61200, 64800, 68400, 72000, 75600, 79200, 82800, 86400, 90000, 93600, 97200, 100800, 104400, 108000, 111600, 115200, 118800, 122400, 126000, 129600, 133200, 136800, 140400, 144000, 147600, 151200, 154800, 158400, 162000, 165600, 169200, 172800, 176400, 180000, 183600, 187200, 190800, 194400, 198000, 201600, 205200, 208800, 212400, 216000, 219600, 223200, 226800, 230400, 234000, 237600, 241200, 244800, 248400, 252000, 255600, 259200, 262800, 266400, 270000, 273600, 277200, 280800, 284400, 288000, 291600, 295200, 298800, 302400, 306000, 309600, 313200, 316800, 320400, 324000, 327600, 331200, 334800, 338400, 342000, 345600, 349200, 352800, 356400, 360000, 363600, 367200, 370800, 374400, 378000, 381600, 385200, 388800, 392400, 396000, 399600, 403200, 406800, 410400, 414000, 417600, 421200, 424800, 428400, 432000, 435600, 439200, 442800, 446400, 450000, 453600]<br>cycle_offset: [None]"
end
style pressure_at_mean_sea_level_enukx fill:#D4A76A
subgraph pressure_at_mean_sea_level_enukx[enukx]
3(["3<br>operation: input"])
click 3 callback "leadtime: [3600, 7200, 10800, 14400, 18000, 21600, 25200, 28800, 32400, 36000, 39600, 43200, 46800, 50400, 54000, 57600, 61200, 64800, 68400, 72000, 75600, 79200, 82800, 86400, 90000, 93600, 97200, 100800, 104400, 108000, 111600, 115200, 118800, 122400, 126000, 129600, 133200, 136800, 140400, 144000, 147600, 151200, 154800, 158400, 162000, 165600, 169200, 172800, 176400, 180000, 183600, 187200, 190800, 194400, 198000, 201600, 205200, 208800, 212400, 216000, 219600, 223200, 226800, 230400, 234000, 237600, 241200, 244800, 248400, 252000, 255600, 259200, 262800, 266400, 270000, 273600, 277200, 280800, 284400, 288000, 291600, 295200, 298800, 302400, 306000, 309600, 313200, 316800, 320400, 324000, 327600, 331200, 334800, 338400, 342000, 345600, 349200, 352800, 356400, 360000, 363600, 367200, 370800, 374400, 378000, 381600, 385200, 388800, 392400, 396000, 399600, 403200, 406800, 410400, 414000, 417600, 421200, 424800, 428400, 432000, 435600, 439200, 442800, 446400, 450000, 453600]<br>cycle_offset: [None]"
4(["4<br>operation: load"])
click 4 callback "leadtime: [3600, 7200, 10800, 14400, 18000, 21600, 25200, 28800, 32400, 36000, 39600, 43200, 46800, 50400, 54000, 57600, 61200, 64800, 68400, 72000, 75600, 79200, 82800, 86400, 90000, 93600, 97200, 100800, 104400, 108000, 111600, 115200, 118800, 122400, 126000, 129600, 133200, 136800, 140400, 144000, 147600, 151200, 154800, 158400, 162000, 165600, 169200, 172800, 176400, 180000, 183600, 187200, 190800, 194400, 198000, 201600, 205200, 208800, 212400, 216000, 219600, 223200, 226800, 230400, 234000, 237600, 241200, 244800, 248400, 252000, 255600, 259200, 262800, 266400, 270000, 273600, 277200, 280800, 284400, 288000, 291600, 295200, 298800, 302400, 306000, 309600, 313200, 316800, 320400, 324000, 327600, 331200, 334800, 338400, 342000, 345600, 349200, 352800, 356400, 360000, 363600, 367200, 370800, 374400, 378000, 381600, 385200, 388800, 392400, 396000, 399600, 403200, 406800, 410400, 414000, 417600, 421200, 424800, 428400, 432000, 435600, 439200, 442800, 446400, 450000, 453600]<br>cycle_offset: [None]"
end
style pressure_at_mean_sea_level_ukvx fill:#D4A76A
subgraph pressure_at_mean_sea_level_ukvx[ukvx]
5(["5<br>operation: input"])
click 5 callback "leadtime: [0, 3600, 7200, 10800, 14400, 18000, 21600, 25200, 28800, 32400, 36000, 39600, 43200]<br>cycle_offset: [None]"
6(["6<br>operation: load"])
click 6 callback "leadtime: [0, 3600, 7200, 10800, 14400, 18000, 21600, 25200, 28800, 32400, 36000, 39600, 43200]<br>cycle_offset: [None]"
end
end
3 --> 0
5 --> 0
4 --> 1
6 --> 1
1 --> 2
3 --> 4
0 --> 4
0 --> 6
5 --> 6

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