[#4899] Fix optional serializer fields in ZaakOptionsSerializer

[vaszig]

Closes #4899

Changes

• Allow optional url fields to receive empty string values

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

[vaszig]

I guess this needs-backport as well, or not since it was added last month.

[sergei-maertens]

I'm such an idiot

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.62%. Comparing base (27a9852) to head (ec54837).
Report is 13 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4903   +/-   ##
=======================================
  Coverage   96.62%   96.62%           
=======================================
  Files         757      757           
  Lines       25942    25942           
  Branches     3417     3417           
=======================================
  Hits        25066    25066           
  Misses        613      613           
  Partials      263      263           

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[vaszig]

I guess this needs-backport as well, or not since it was added last month.

@sergei-maertens the product_url was added last month from Robin and it's in the Changelog for v3.0, so I am removing the needs-backport label.