Harmonize Path with empty filename between windows and linux.

[DrChainsaw]

This fixes inconsistent mv behaviour between windows and linux when filename is an empty string:

julia> t = maketree("root" => [(name="", value=1), (name="a", value=2)])
root/
├─  (Int64)
└─ a (Int64)

Before on linux:

julia> mv(t, r"(.*)", s"\1") # Noop mv
root/
└─ a (Int64)

After on linux (and before on windows):

julia> mv(t, r"(.*)", s"\1")
root/
├─  (Int64)
└─ a (Int64)

Not sure if empty names should even be allowed, but since they are, then I think mv should not silently delete them.