MudTable: Add nullable annotation.

[ScarletKuro]

Description

Part of this issue #6535
Fixes: #7629

NB! There probably will be adjustment in future for the annotation. Currently some null checks can be excessive in places where I wasn't sure.

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.
• I've added relevant tests.

[codecov]

Codecov Report

Attention: Patch coverage is 70.40359% with 66 lines in your changes are missing coverage. Please review.

Project coverage is 90.12%. Comparing base (28bc599) to head (e8fd3d8).
Report is 170 commits behind head on dev.

Files	Patch %	Lines
src/MudBlazor/Components/Table/MudTr.razor.cs	51.42%	0 Missing and 17 partials ⚠️
src/MudBlazor/Components/Table/MudTable.razor.cs	62.50%	3 Missing and 6 partials ⚠️
...Blazor/Components/Table/MudTableSortLabel.razor.cs	66.66%	2 Missing and 5 partials ⚠️
src/MudBlazor/Components/Table/TableContext.cs	72.72%	2 Missing and 4 partials ⚠️
.../MudBlazor/Components/Table/MudTableGroupRow.razor	0.00%	0 Missing and 4 partials ⚠️
...MudBlazor/Components/Table/MudTemplateColumn.razor	0.00%	0 Missing and 4 partials ⚠️
...dBlazor/Components/Table/MudTableGroupRow.razor.cs	81.25%	0 Missing and 3 partials ⚠️
.../MudBlazor/Components/Table/MudTablePager.razor.cs	88.23%	0 Missing and 2 partials ⚠️
src/MudBlazor/Components/Table/MudTr.razor	0.00%	0 Missing and 2 partials ⚠️
...Blazor/Extensions/TableButtonPositionExtensions.cs	77.77%	0 Missing and 2 partials ⚠️
... and 10 more

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8926      +/-   ##
==========================================
+ Coverage   89.82%   90.12%   +0.29%     
==========================================
  Files         412      426      +14     
  Lines       11878    12310     +432     
  Branches     2364     2422      +58     
==========================================
+ Hits        10670    11094     +424     
+ Misses        681      669      -12     
- Partials      527      547      +20     

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