Enable nullability in WindowsFormsDesignerOptionService

[gpetrou]

Proposed changes

• Enable nullability in WindowsFormsDesignerOptionService.

Microsoft Reviewers: Open in CodeFlow

[Tanya-Solyanik]

Error message
Assert.Empty() Failure: Collection was not empty\r\nCollection: [[]]

Stack trace
at System.Windows.Forms.Design.Tests.WindowsFormsDesignerOptionServiceTests.WindowsFormsDesignerOptionService_Options_GetNullDesignerOptions_ReturnsExpected() in D:\a_work\1\s\src\System.Windows.Forms.Design\tests\UnitTests\System\Windows\Forms\Design\WindowsFormsDesignerOptionServiceTests.cs:line 45
at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

[Tanya-Solyanik]

@gpetrou - this PR is almost ready, is it OK with you if we take it over and finish up?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.24042%. Comparing base (4bcc8a9) to head (bf6b89e).
Report is 211 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #13059          +/-   ##
====================================================
- Coverage   96.95044%   62.24042%   -34.71002%     
====================================================
  Files           1225        1556         +331     
  Lines         357134      159202      -197932     
  Branches        5537       14854        +9317     
====================================================
- Hits          346243       99088      -247155     
- Misses         10049       59363       +49314     
+ Partials         842         751          -91     

Flag	Coverage Δ
Debug	62.24042% <100.00000%> (-34.71003%)	⬇️
integration	11.31896% <0.00000%> (?)
production	40.37199% <100.00000%> (?)
test	95.66723% <ø> (-1.28321%)	⬇️
unit	37.72444% <100.00000%> (?)

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