xrCore: Fix undefined behavior passing `nullptr` to `dlclose` in `ModuleLookup::Close()`

Created by: AMS21

It's undefined behavior to call dlclose with with null. But in the case of our handle being null we gladly don't have to do anything so we can just return.

UBSAN message:

/mnt/data/dev/xray-16/src/xrCore/ModuleLookup.cpp:71:12: runtime error: null pointer passed as argument 1, which is declared to never be null