MudBaseInput: 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

Attention: Patch coverage is 68.75000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 89.85%. Comparing base (28bc599) to head (2511635).
Report is 18 commits behind head on dev.

Files	Patch %	Lines
src/MudBlazor/Base/MudBaseInput.cs	68.75%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8635      +/-   ##
==========================================
+ Coverage   89.82%   89.85%   +0.02%     
==========================================
  Files         412      414       +2     
  Lines       11878    11913      +35     
  Branches     2364     2364              
==========================================
+ Hits        10670    10705      +35     
+ Misses        681      677       -4     
- Partials      527      531       +4     

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