🧪 Add tests for unused schema fields

[ubmarco]

Currently, if there is no schema defined for an extra option, the field is not nullable as this resembles the behavior of SN<6.
If the field is unset, the value will be an empty string.

If there is a schema defined, the field is nullable because, if the field is given, it must coerce to the given type and, if it is not given, it is stored as None in Python and null in JSON.

Adding tests for these 2 cases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.96%. Comparing base (4e10030) to head (7b466cd).
⚠️ Report is 192 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1572      +/-   ##
==========================================
+ Coverage   86.87%   87.96%   +1.08%     
==========================================
  Files          56       70      +14     
  Lines        6532     9638    +3106     
==========================================
+ Hits         5675     8478    +2803     
- Misses        857     1160     +303     

Flag	Coverage Δ
pytests	87.96% <ø> (+1.08%)	⬆️

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