MudAvatar: Add nullable annotation

[ScarletKuro]

Description

Part of this issue #6535

No open pull requests for MudAvatar.

How Has This Been Tested?

No tests required in case of MudAvatar

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.
• I've added relevant tests.

[ScarletKuro]

Included MudComponentBase annotation as well, should have done it earlier since all components inherit from it.

few nits (future?) for MudComponentBase: UserAttributes should be IReadOnlyDictionary since it should be immutable.
Instead of UserAttributes.Contains("XYZ") -> UserAttributes["XYZ"] it would b nice to use TryGetValue(key, ref) since it's more optimized. ContainsKey + Item is about 40% slower.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10 🎉

Comparison is base (eaf5d78) 91.37% compared to head (7b794f2) 91.47%.

❗ Current head 7b794f2 differs from pull request most recent head faba67a. Consider uploading reports for the commit faba67a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6543      +/-   ##
==========================================
+ Coverage   91.37%   91.47%   +0.10%     
==========================================
  Files         397      393       -4     
  Lines       14909    14875      -34     
==========================================
- Hits        13623    13607      -16     
+ Misses       1286     1268      -18     

Impacted Files	Coverage Δ
src/MudBlazor/Components/Avatar/MudAvatar.razor.cs	100.00% <ø> (ø)
src/MudBlazor/Base/MudComponentBase.cs	100.00% <100.00%> (ø)
...udBlazor/Components/Avatar/MudAvatarGroup.razor.cs	100.00% <100.00%> (ø)

... and 19 files with indirect coverage changes

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[ScarletKuro]

If no objections, then good to merge.

[ScarletKuro]

Rebased and fixed the conflicts.