fix(path): avoid shell fallback for unknown users
Problem:
Expanding an unknown ~user path falls back to the shell. In a fast event,
this re-enters the event loop and aborts Nvim.
Solution:
Leave unknown users unexpanded when the system account lookup fails.
(cherry picked from commit 9d3edf345f)
Co-authored-by: MAAZIZ Adel Ayoub <adelayoub.maaziz@gmail.com>