Handle when DestroyStructure API passes null to the IL marshaller

[AaronRobinsonMSFT]

During the clean up phase for IL marshallers, managed value types passed as nullptr can cause failures.
This is an issue with field marshallers of nested non-blittable types only.

runtime/src/coreclr/vm/marshalnative.cpp

Lines 228 to 238 in f24b363

	else if (th.HasLayout())
	{
	MethodDesc* structMarshalStub;
	{
	GCX_PREEMP();
	structMarshalStub = NDirect::CreateStructMarshalILStub(th.GetMethodTable());
	}
	MarshalStructViaILStub(structMarshalStub, nullptr, ptr, StructMarshalStubs::MarshalOperation::Cleanup);
	}

Fixes #61839

/cc @jkoritzinsky @elinor-fung

[AaronRobinsonMSFT]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/1497375119