Added null ref check into CodeGenHelpers.cs

[andreyshade]

Fixed NullRefference exception in CodeGenHelpers after package update
(0,0): error - System.NullReferenceException: Object reference not set to an instance of an object.|| at Unity.Netcode.Editor.CodeGen.CodeGenHelpers.IsSubclassOf(TypeReference typeReference, TypeReference baseClass)

[unity-cla-assistant]

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[ShadauxCat]

Hi @andreyshade! Thank you for your contribution!

For reasons related to our internal CI process, we're unable to directly incorporate PRs from the community. I've opened a new PR with your fix as #2581 and we're targeting including this fix in our upcoming 1.5 release.

Since I've moved the PR to that new PR, I'm going to close this one. Thank you again for your contribution.