[OF#3576] Fix empty label fields

[SilviaAmAm]

Fixes open-formulieren/open-forms#3576

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bc557b9) 71.75% compared to head (18f0838) 55.87%.

❗ Current head 18f0838 differs from pull request most recent head 7f91177. Consider uploading reports for the commit 7f91177 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #591       +/-   ##
===========================================
- Coverage   71.75%   55.87%   -15.88%     
===========================================
  Files         212      208        -4     
  Lines        4301     3762      -539     
  Branches     1164      684      -480     
===========================================
- Hits         3086     2102      -984     
- Misses       1183     1466      +283     
- Partials       32      194      +162     

see 136 files with indirect coverage changes

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