Fixes to support hyperion-blueapi

[rtuck99]

Change BartRobot to use a sentinel instead of None for the empty location

Fixes DiamondLightSource/mx-bluesky#1364

See also mx-bluesky changes:

• Ability to launch blueapi and run Hyperion plans mx-bluesky#1389

Instructions to reviewer on how to test:

1. Do thing x
2. Confirm thing y happens

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.10%. Comparing base (7cdf4cf) to head (ba979d5).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1760   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files         281      281           
  Lines       10549    10550    +1     
=======================================
+ Hits        10455    10456    +1     
  Misses         94       94           

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