[#4516] Fix import of forms with Objects API registration backends without any group set

[Viicos]

Closes #4516

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

• 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 96.57%. Comparing base (ac6a7c2) to head (578d43f).
Report is 519 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4522   +/-   ##
=======================================
  Coverage   96.57%   96.57%           
=======================================
  Files         719      719           
  Lines       24089    24088    -1     
  Branches     2863     2866    +3     
=======================================
+ Hits        23263    23264    +1     
+ Misses        562      560    -2     
  Partials      264      264           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sergei-maertens]

Will merge and backport tomorrow!