MudDivider: Add nullable annotation.

[ScarletKuro]

Description

Part of this issue #6535

How Has This Been Tested?

No tests required in case of MudDivider

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: 100.00% and no project coverage change.

Comparison is base (50da41d) 90.55% compared to head (bedd5cd) 90.55%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #6641   +/-   ##
=======================================
  Coverage   90.55%   90.55%           
=======================================
  Files         399      399           
  Lines       13610    13610           
=======================================
  Hits        12324    12324           
  Misses       1286     1286           

Impacted Files	Coverage Δ
src/MudBlazor/Components/Divider/MudDivider.razor	100.00% <100.00%> (ø)

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