MudInputAdornment: Add nullable annotation.

[ScarletKuro]

Description

Part of this issue #6535

How Has This Been Tested?

No tests required.

Type 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)
• Documentation (fix or improvement to the website or code docs)

Checklist

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

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.20%. Comparing base (6759d4a) to head (c75faaf).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10203      +/-   ##
==========================================
- Coverage   91.21%   91.20%   -0.01%     
==========================================
  Files         411      411              
  Lines       12502    12502              
  Branches     2436     2436              
==========================================
- Hits        11404    11403       -1     
  Misses        554      554              
- Partials      544      545       +1     

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