Fix explicitly requested empty protocol

[ap--]

This PR enforces that when a user explicitly requests protocol="" (empty) when creating a UPath, they are guaranteed to get a PosixUPath or WindowsUPath instance.

import upath

upath.UPath("s3://bucket/file", protocol="")  # now raises a ValueError
upath.UPath("file:///abc", protocol="")  # also raises a ValueError 
upath.UPath("/a/b/c")  # returns a PosixUPath / WindowsUPath

# whereas:

upath.UPath("file:///abc", protocol="file")  # returns  FilePath
upath.UPath("file:///abc")  # returns FilePath