Add unit tests for DesignerVerbToolStripMenuItem

[MelonWang1]

Related #10773

Proposed changes

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

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.80128%. Comparing base (b91747d) to head (2bf791f).
Report is 10 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12566         +/-   ##
===================================================
+ Coverage   75.77766%   75.80128%   +0.02362%     
===================================================
  Files           3161        3163          +2     
  Lines         636074      636389        +315     
  Branches       46989       46997          +8     
===================================================
+ Hits          482002      482391        +389     
+ Misses        150614      150530         -84     
- Partials        3458        3468         +10     

Flag	Coverage Δ
Debug	75.80128% <100.00000%> (+0.02362%)	⬆️
integration	18.07202% <ø> (-0.12076%)	⬇️
production	49.38349% <ø> (+0.02680%)	⬆️
test	97.05071% <100.00000%> (+0.00260%)	⬆️
unit	46.65590% <ø> (+0.11954%)	⬆️

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[ricardobossan]

All LGTM!

[Tanya-Solyanik]

Thank you!