Fix unixify when beginning with current directory.

Fix unixify when beginning with current directory.

VMS::Filespec::unixify has been truncating its return value and
returning early when the input begins with [] meaning the current
directory.  If there was nothing else, we've been getting the right
answer:

    [] --> ./

but if there was a file portion of the name it's been getting
omitted:

    []foo.txt --> ./

which is now fixed.  Looks like it's been broken since inception
in 5.002, though only with the specific [] case and not if there
was an explicit device or directory name.