Add unit tests for ToolStripActionList

[MelonWang1]

Related #10773

Proposed changes

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

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.75632%. Comparing base (dc4314e) to head (ed7b62e).
Report is 8 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12525         +/-   ##
===================================================
+ Coverage   75.74163%   75.75632%   +0.01469%     
===================================================
  Files           3157        3158          +1     
  Lines         636098      636207        +109     
  Branches       47003       47005          +2     
===================================================
+ Hits          481791      481967        +176     
+ Misses        150846      150777         -69     
- Partials        3461        3463          +2     

Flag	Coverage Δ
Debug	75.75632% <100.00000%> (+0.01469%)	⬆️
integration	18.18374% <ø> (-0.00149%)	⬇️
production	49.33317% <ø> (+0.02378%)	⬆️
test	97.04621% <100.00000%> (+0.00091%)	⬆️
unit	46.54537% <ø> (+0.02344%)	⬆️

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[ricardobossan]

Other than small comment, LGTM