Add unit tests for TextBoxActionList

[MelonWang1]

Related #10773

Proposed changes

• Add unit test TextBoxActionListTests.cs for public properties and method of the TextBoxActionList.
• Enable nullability in TextBoxActionList.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.38943%. Comparing base (66894e2) to head (c928d9a).
Report is 3 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12198         +/-   ##
===================================================
+ Coverage   75.34760%   75.38943%   +0.04183%     
===================================================
  Files           3099        3100          +1     
  Lines         634429      634451         +22     
  Branches       46874       46874                 
===================================================
+ Hits          478027      478309        +282     
+ Misses        152989      152732        -257     
+ Partials        3413        3410          -3     

Flag	Coverage Δ
Debug	75.38943% <100.00000%> (+0.04183%)	⬆️
integration	17.99846% <ø> (+0.07047%)	⬆️
production	48.76677% <ø> (+0.09176%)	⬆️
test	97.02681% <100.00000%> (-0.00011%)	⬇️
unit	45.80146% <ø> (+0.00844%)	⬆️

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[Tanya-Solyanik]

Thank you!