bug#36281: 27.0.50; ediff-directories() ends up with a wrong-type-argument lisp error

[Markus Weber]

Hi,

probably with commit 963d4e2, ediff.el seems to be mixed up.

Reproducible: always, with current builds on Debian and mingw-w64.

Stack:
> Debugger entered--Lisp error: (wrong-type-argument stringp nil)
>  string-match("^/tmp_mnt/" nil)
>  abbreviate-file-name(nil)
>  ediff-abbreviate-file-name(nil)
>  ediff-draw-dir-diffs(((#<buffer *Ediff Session Group Panel<2>*>)))
>  ...

Many thanks!