MudFormComponent: Add nullable annotation.

[ScarletKuro]

Description

Part of this issue #6535
To continue annotation need to annotate some main classes.
It also annotates the Converter which is important for the component.

There is one change additional change that it check that error is not null but also not empty (""), doesn't look like it would be desired to have empty lines in the error list.

How Has This Been Tested?

Unit tests

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: 94.33% and project coverage change: -0.01 ⚠️

Comparison is base (50da41d) 90.55% compared to head (b2c2f84) 90.54%.

❗ Current head b2c2f84 differs from pull request most recent head 35ef48a. Consider uploading reports for the commit 35ef48a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6608      +/-   ##
==========================================
- Coverage   90.55%   90.54%   -0.01%     
==========================================
  Files         399      399              
  Lines       13610    13615       +5     
==========================================
+ Hits        12324    12328       +4     
- Misses       1286     1287       +1     

Impacted Files	Coverage Δ
src/MudBlazor/Extensions/ExpressionExtensions.cs	100.00% <ø> (ø)
...MudBlazor/Utilities/BindingConverters/Converter.cs	74.19% <ø> (ø)
src/MudBlazor/Base/MudFormComponent.cs	78.43% <94.33%> (+0.02%)	⬆️

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[ScarletKuro]

Do not merge yet.
Ready now.