reading persistent_storage may return a nil object so updated the check ...

...to prevent file comparison from failing.


Background: I ran into a case where read_persistent_storage() returns a nil object as the file path, so when the identical_files(persistent_storage, location) method executes, the Pathname.new(nil) call throws an exception.

So I've added a check well before that on the persistent_storage file name validation. Hope this is alright.