[StyleCleanUp] Use is null check (IDE0041)

[h3xds1nz]

Fixes #10612

Description

Avoids using ReferenceEquals to check each object and uses is null check instead.

• In case of WeakRefKey and DpiScale, the boxing casts are removed as they're both non-nullable structs.

Customer Impact

Cleaner codebase for developers, should reduce some boxes and branches.

Regression

No.

Testing

Local testing/build.

Risk

Should be low, mostly analyzer fixes but I did adjust some to remove double-checks on the same field.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.

Project coverage is 11.71271%. Comparing base (b6914d7) to head (e1cee12).

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #10613         +/-   ##
===================================================
+ Coverage   11.45872%   11.71271%   +0.25398%     
===================================================
  Files           3214        3214                 
  Lines         648458      648398         -60     
  Branches       71511       71501         -10     
===================================================
+ Hits           74305       75945       +1640     
+ Misses        572989      571199       -1790     
- Partials        1164        1254         +90     

Flag	Coverage Δ
Debug	11.71271% <0.00000%> (+0.25398%)	⬆️

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