MudBadge: Add nullable annotation.

[ScarletKuro]

Description

Part of this issue #6535

How Has This Been Tested?

No tests required in case of MudBadge

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

Patch coverage: 100.00% and project coverage change: +0.29 🎉

Comparison is base (e23518e) 91.17% compared to head (89902e4) 91.46%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6554      +/-   ##
==========================================
+ Coverage   91.17%   91.46%   +0.29%     
==========================================
  Files         392      393       +1     
  Lines       14836    14874      +38     
==========================================
+ Hits        13527    13605      +78     
+ Misses       1309     1269      -40     

Impacted Files	Coverage Δ
src/MudBlazor/Components/Badge/MudBadge.razor.cs	100.00% <ø> (ø)
src/MudBlazor/Components/Badge/MudBadge.razor	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[ScarletKuro]

@henon