[#4853] Fix non required fields in registration backends serializers

[vaszig]

Closes #4853

Changes

• Updated serializers with non required fields. When we send empty strings to the backend we need allow_blank=True

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 96.58%. Comparing base (753691c) to head (2b3967c).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4924   +/-   ##
=======================================
  Coverage   96.58%   96.58%           
=======================================
  Files         757      757           
  Lines       25783    25783           
  Branches     3390     3390           
=======================================
  Hits        24902    24902           
  Misses        616      616           
  Partials      265      265           

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[sergei-maertens]

Backports:

• stable/2.8.x: fa35b36

Not backporting to older versions, since the UI/UX rework was started in 2.8