CI Build Fix

[ssparach]

Summary of the pull request

Enable nullable property in relevant classes

References and relevant issues

Detailed description of the pull request / Additional comments

Resolves below errors observed in CI builds:
"D:\a\devhome\devhome\DevHome.sln" (default target) (1) ->
"D:\a\devhome\devhome\src\DevHome.csproj.metaproj" (default target) (2) ->
"D:\a\devhome\devhome\src\DevHome.csproj" (default target) (11) ->
"D:\a\devhome\devhome\tools\Customization\DevHome.FileExplorerSourceControlIntegration\DevHome.FileExplorerSourceControlIntegration.csproj" (default target) (15:2) ->
(CoreCompile target) ->
D:\a\devhome\devhome\tools\Customization\DevHome.FileExplorerSourceControlIntegration\Services\RepositoryTracking.cs(32,80): error CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. [D:\a\devhome\devhome\tools\Customization\DevHome.FileExplorerSourceControlIntegration\DevHome.FileExplorerSourceControlIntegration.csproj]
D:\a\devhome\devhome\tools\Customization\DevHome.FileExplorerSourceControlIntegration\Services\RepositoryTracking.cs(34,37): error CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. [D:\a\devhome\devhome\tools\Customization\DevHome.FileExplorerSourceControlIntegration\DevHome.FileExplorerSourceControlIntegration.csproj]
D:\a\devhome\devhome\tools\Customization\DevHome.FileExplorerSourceControlIntegration\Services\RepoStoreOptions.cs(20,19): error CS8632: The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. [D:\a\devhome\devhome\tools\Customization\DevHome.FileExplorerSourceControlIntegration\DevHome.FileExplorerSourceControlIntegration.csproj]

Validation steps performed

Local Builds (both VS and using build script in Developer Command Prompt to build msix release) are successful

PR checklist

• Closes #xxx
• Tests added/passed
• Documentation updated