DataGrid : Fix sorting when there is Header row (#7645)

[timlunev]

Description

MudDataGrid has an issue with the HeaderCell component when the Header is added and there is no Column on a sort.
This PR fixes #7645

How Has This Been Tested?

A new DataGrid unit test was added to verify if sorting does not throw NullReferenceException and if sorting is still working as expected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• 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.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.08%. Comparing base (5929972) to head (78283c7).
Report is 1 commits behind head on dev.

Files	Patch %	Lines
.../MudBlazor/Components/DataGrid/HeaderCell.razor.cs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8504      +/-   ##
==========================================
+ Coverage   89.03%   89.08%   +0.04%     
==========================================
  Files         411      411              
  Lines       12175    12173       -2     
  Branches     2431     2431              
==========================================
+ Hits        10840    10844       +4     
+ Misses        799      795       -4     
+ Partials      536      534       -2     

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[ScarletKuro]

@henon I suggest to merge it without @tjscience review as it just fixed a null exception.

[henon]

Thanks @timlunev