Avoid empty variables being marked as geometry fields (objects API registration)

[sergei-maertens]

Closes #4978 (partially)

Changes

Variables with empty keys are not usable so these are now excluded from the isGeometry checks.

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Dockerfile/scripts

  • Updated the Dockerfile with the necessary scripts from the ./bin folder

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.66%. Comparing base (7f8fd3b) to head (0c3da88).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4988      +/-   ##
==========================================
- Coverage   96.66%   96.66%   -0.01%     
==========================================
  Files         761      761              
  Lines       25957    25957              
  Branches     3393     3393              
==========================================
- Hits        25092    25091       -1     
  Misses        601      601              
- Partials      264      265       +1     

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