dont use `false` as cache key for non utf8 path in normalizePath

since `json_encode` returns `false` if it's input isn't utf8, all non utf8 paths passed to normalizePath will currently return the same cached result.

Fixing this makes working with non utf8 storages a *little* bit more possible for apps

Signed-off-by: Robin Appelman <robin@icewind.nl>