Enable nullable reference types in GenerateNativeAotEnvironmentAssemblerSources.cs

[Copilot]

outputFile (declared ITaskItem?) was dereferenced without a null check, and ! null-forgiving operators were used — both violating repo conventions.

• Add #nullable enable
• Add null guard after FindOutputFile returns, logging an error and returning false
• Remove ! from environmentLlFilePath (now guaranteed non-null after the guard)

ITaskItem? outputFile = GenerateNativeAotLibraryLoadAssemblerSources.FindOutputFile (OutputSources, abi: abi, rid: RID);
if (outputFile == null) {
	Log.LogError ($"Could not find output file for ABI '{abi}' and RID '{RID}'");
	return false;
}

[Copilot]

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