tests: utils: Allow NULL strings to be equal to empty file in virTestCompareToFile
tests: utils: Allow NULL strings to be equal to empty file in virTestCompareToFile

Treat an NULL string equivalent to an empty string in
virTestCompareToFile so that callers don't need to add additional logic
in case when a test produces no output.

jfehlig/libvirt,jfehlig/libvirt,jardasgit/libvirt,jardasgit/libvirt,olafhering/libvirt,zippy2/libvirt,crobinso/libvirt,libvirt/libvirt,andreabolognani/libvirt,eskultety/libvirt,nertpinx/libvirt,zippy2/libvirt,datto/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,libvirt/libvirt,eskultety/libvirt,andreabolognani/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,olafhering/libvirt,datto/libvirt,nertpinx/libvirt,crobinso/libvirt,fabianfreyer/libvirt,eskultety/libvirt,libvirt/libvirt,olafhering/libvirt,nertpinx/libvirt,datto/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,zippy2/libvirt,datto/libvirt,eskultety/libvirt,eskultety/libvirt,olafhering/libvirt,datto/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,crobinso/libvirt,fabianfreyer/libvirt,crobinso/libvirt,zippy2/libvirt,jardasgit/libvirt,jardasgit/libvirt