added guard against missing implicit helps in custom compose packages

[tclose]

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Summary

In custom compose plugins, there may not be a way to implicitly pull out help strings, in which case they will be None by default in ensure_field_objects, but this was not being checked.

Checklist

• I have added tests to cover my changes (if necessary)
• I have updated documentation (if necessary)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.74%. Comparing base (964793d) to head (af54f3d).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #819      +/-   ##
==========================================
+ Coverage   88.71%   88.74%   +0.02%     
==========================================
  Files          84       84              
  Lines       17891    17890       -1     
  Branches     1555     3483    +1928     
==========================================
+ Hits        15872    15876       +4     
+ Misses       1650     1643       -7     
- Partials      369      371       +2     

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