MudMenu: Add nullable annotation.

[ScarletKuro]

Description

Part of this issue #6535

How Has This Been Tested?

No tests required.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change, improve null diagnostic warnings)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.86%. Comparing base (28bc599) to head (f4d723c).
Report is 17 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8632      +/-   ##
==========================================
+ Coverage   89.82%   89.86%   +0.03%     
==========================================
  Files         412      414       +2     
  Lines       11878    11911      +33     
  Branches     2364     2364              
==========================================
+ Hits        10670    10704      +34     
+ Misses        681      676       -5     
- Partials      527      531       +4     

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