In some cases, the Tk library may have been directly compiled in or
bundled into the main executable by the time Pillow runs, in which case
__file__ isn't available (nor would it make sense to use, anyway).
If __file__ is missing, then set the library path to None and rely on
our Tk loader being able to find the function pointers within the main
binary - we know we probably have it because we've managed to import it
already.