MudDataGrid: Fix GroupExpanded when using ServerData

[ghost]

Description

Fixes GroupExpanded not working regression when using ServerData, caused by PR #6660.

How Has This Been Tested?

unit | visually

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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (09e619a) 90.45% compared to head (388392b) 90.45%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #6667   +/-   ##
=======================================
  Coverage   90.45%   90.45%           
=======================================
  Files         399      399           
  Lines       13632    13632           
=======================================
  Hits        12331    12331           
  Misses       1301     1301           

Impacted Files	Coverage Δ
...MudBlazor/Components/DataGrid/MudDataGrid.razor.cs	90.88% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[henon]

Thanks @alfadelta88, you have been a great help! FYI @tjscience