API export null header

[jnegrel]

The API export throws an error when an instrument has a sensor_type with a "null" "column_heading".
Suggestion to add a test and fall back on sensor name when "null" is encountered

[squaregoldfish]

Alternatively, have a check on app startup that checks to ensure that all sensor_types have a column heading. If we decide that it's mandatory. This would be related to #570