Only uninitialise OpenXR on destruct if it was initialized

[BastiaanOlij]

The OpenXRInterface is constructed and added to the XRServer to make it available, when destructing it was calling it's uninitialise function to ensure all is cleaned up however if OpenXR was never even enabled that function gave some error spam.

Now only calling it if the OpenXRInterface was previously initialized to ensure it is only run when applicable.

[fire]

A paper review of this makes logical sense that we should check if a thing is initialized before uninitializing it.

[fire]

@BastiaanOlij It's a bit weird that you can have OpenXR that is created but uninitialized.

Calling the public function virtual void uninitialize() override; will cause the error spam too. Maybe the check should be inside of uninitialized?

[akien-mga]

Thanks!