[#4627] Fix acting_subject_claim KeyError in eHerkenning via OIDC

[vaszig]

Closes #4627

Changes

• Added regression test
• Make acting_subject_claim optional since we do not know if it will exist in the broker's response

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

• 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 96.68%. Comparing base (262d84e) to head (f54f7cd).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4630   +/-   ##
=======================================
  Coverage   96.68%   96.68%           
=======================================
  Files         732      732           
  Lines       24649    24649           
  Branches     3229     3229           
=======================================
  Hits        23832    23832           
  Misses        564      564           
  Partials      253      253           

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[sergei-maertens]

Backport: 7515293