Enable nullable reference types for System.Windows.Forms.Primitives

[RussKie]

Based on work started by @sharwell in #2676, renders it obsolete.

Microsoft Reviewers: Open in CodeFlow

[RussKie]

@vladimir-krestov @SergeySmirnov-Akvelon @Ryuugamine please note of NRT annotations for the accessibility-related API.

[JeremyKuhne]

This one worries me- it was clearly object?[] historically. That may have been wrong, but I would want the functional change taken out of this so we discuss and track for regression purposes if we take this change.

[RussKie]

That was my guess work when I annotated AccessibleObject in #3273 based on the use. Now having annotated the imported interface object[]? looked more appropriate.
Now I'm re-reading docs, and it looks like it may be returning an empty array, if there is no selection - https://docs.microsoft.com/en-us/dotnet/api/system.windows.automation.provider.iselectionprovider.getselection?view=netcore-3.1#remarks

[RussKie]

On further review, the current annotation is consistent with the existing implementation:

winforms/src/System.Windows.Forms/src/System/Windows/Forms/AccessibleObject.cs

Line 691 in d6d0f86

internal virtual UiaCore.IRawElementProviderSimple[]? GetSelection() => null;

winforms/src/System.Windows.Forms.Primitives/src/Interop/UiaCore/Interop.ILegacyIAccessibleProvider.cs

Line 42 in 423b2bc

object[] /* IRawElementProviderSimple[] */ GetSelection();

So I'm leaving it as is.

@vladimir-krestov please review the API definitions for correctness (i.e. is null appropriate or has to be empty array?) and update, if necessary.

[codecov]

Codecov Report

Merging #3451 into master will increase coverage by 31.91179%.
The diff coverage is 100.00000%.

@@                 Coverage Diff                  @@
##              master       #3451          +/-   ##
====================================================
+ Coverage   66.58567%   98.49746%   +31.91179%     
====================================================
  Files           1338         443         -895     
  Lines         501683      248046      -253637     
  Branches       40864        4088       -36776     
====================================================
- Hits          334049      244319       -89730     
+ Misses        162085        3015      -159070     
+ Partials        5549         712        -4837     

Flag	Coverage Δ
#Debug	98.49746% <100.00000%> (+31.91179%)	⬆️
#production	?
#test	98.49746% <100.00000%> (+0.00011%)	⬆️

[RussKie]

/azp run

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