storage: file: Tolerate NULL src when uninitializing the backend
storage: file: Tolerate NULL src when uninitializing the backend

Allow de-init of null storage sources.

crobinso/libvirt,andreabolognani/libvirt,rlaager/libvirt,crobinso/libvirt,elmarco/libvirt,taget/libvirt,datto/libvirt,eskultety/libvirt,rlaager/libvirt,agx/libvirt,rlaager/libvirt,crobinso/libvirt,VenkatDatta/libvirt,eskultety/libvirt,agx/libvirt,datto/libvirt,nertpinx/libvirt,agx/libvirt,zippy2/libvirt,zippy2/libvirt,jfehlig/libvirt,jardasgit/libvirt,jardasgit/libvirt,eskultety/libvirt,eskultety/libvirt,jardasgit/libvirt,nertpinx/libvirt,elmarco/libvirt,jardasgit/libvirt,libvirt/libvirt,jfehlig/libvirt,libvirt/libvirt,elmarco/libvirt,zippy2/libvirt,rlaager/libvirt,agx/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,olafhering/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,taget/libvirt,libvirt/libvirt,eskultety/libvirt,fabianfreyer/libvirt,taget/libvirt,zippy2/libvirt,andreabolognani/libvirt,datto/libvirt,taget/libvirt,olafhering/libvirt,shugaoye/libvirt,datto/libvirt,shugaoye/libvirt,elmarco/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,agx/libvirt,shugaoye/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,rlaager/libvirt,olafhering/libvirt,libvirt/libvirt,taget/libvirt,andreabolognani/libvirt,nertpinx/libvirt,crobinso/libvirt,jardasgit/libvirt,jfehlig/libvirt,elmarco/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,shugaoye/libvirt,olafhering/libvirt,datto/libvirt