Issue/4765 default value migration fail

[robinmolen]

Closes #4765

Changes

Fixes bug in the fix_empty_default_value migration transformer

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • [X] Checked copying a form
  • [X] Checked import/export of a form
  • [X] Config checks in the configuration overview admin page
  • [X] Problem detection in the admin email digest is handled

• Release management

  • [x] I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • [X] Ran ./bin/makemessages_js.sh
  • [X] Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • [X] Commit messages refer to the relevant Github issue
  • [X] Commit messages explain the "why" of change, not the how

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.56%. Comparing base (9c49379) to head (84f406a). Report is 4 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4766 +/- ## ======================================= Coverage 96.56% 96.56% ======================================= Files 746 746 Lines 25205 25208 +3 Branches 3320 3321 +1 ======================================= + Hits 24340 24343 +3 Misses 602 602 Partials 263 263 ```

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