Fix for string form fields

[SchrodingersGat]

• replace null values with empty strings
• Ensure that strings are not replaced with null values sometimes

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[wolflu05]

We have to carefully consider this change, I'm not really sure if this does not break something.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.05%. Comparing base (a76ec0a) to head (cf5665c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10814   +/-   ##
=======================================
  Coverage   88.04%   88.05%           
=======================================
  Files        1281     1281           
  Lines       57625    57628    +3     
  Branches     1997     1997           
=======================================
+ Hits        50736    50742    +6     
+ Misses       6392     6389    -3     
  Partials      497      497           

Flag	Coverage Δ
backend	89.67% <100.00%> (+<0.01%)	⬆️

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Components	Coverage Δ
Backend Apps	92.00% <ø> (ø)
Backend General	93.79% <ø> (ø)
Frontend	70.39% <ø> (-0.01%)	⬇️

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[SchrodingersGat]

@wolflu05 I have added some extra checks here, it will only overwrite a null value if the serializer does not allow null values