Don't set default value to None after applying compat transformation

[manuelma]

Re #395

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov[bot]]

Codecov Report

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

Project coverage is 78.24%. Comparing base (9331bab) to head (dbbfd39).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #396 +/- ## ========================================== - Coverage 78.25% 78.24% -0.01% ========================================== Files 75 75 Lines 9511 9503 -8 Branches 2014 2011 -3 ========================================== - Hits 7443 7436 -7 Misses 1735 1735 + Partials 333 332 -1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.