Avoid these warnings on the console:
```
WARN (bpy.rna): source/blender/python/intern/bpy_rna.cc:1339
pyrna_enum_to_py: current value '0' matches no enum in 'Scene', 'Scene',
'flamenco_job_type'
```
The solution was two-fold:
- Use a non-empty string as the identifier for the 'Select a Job Type'
choice.
- Give the property a default value.
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