MudButtonGroup: Add nullable annotation.

[ScarletKuro]

Description

Part of this issue #6535

Used re-sharpers new analyze for reducing the number of lookups in collections
from

if(!UserAttributes.ContainsKey("role")) 
  UserAttributes.Add("role", "group");

to

UserAttributes.TryAdd("role", "group");

Furthermore, we agreed to apply a code style change when applicable: new line after attribute. Also, fix the indentation for CssBuilder.

How Has This Been Tested?

No tests required in case of MudButtonGroup

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 project coverage change: -0.01 ⚠️

Comparison is base (50da41d) 90.55% compared to head (7e42fc9) 90.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6640      +/-   ##
==========================================
- Coverage   90.55%   90.55%   -0.01%     
==========================================
  Files         399      399              
  Lines       13610    13609       -1     
==========================================
- Hits        12324    12323       -1     
  Misses       1286     1286              

Impacted Files	Coverage Δ
...Blazor/Components/ButtonGroup/MudButtonGroup.razor	100.00% <100.00%> (ø)
...zor/Components/ButtonGroup/MudButtonGroup.razor.cs	100.00% <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 Kuro