bug#54294: 29.0.50; Emacs can't unlock files with --no-build-details

[Glenn Morris]

Philipp Stephani wrote:

> emacs -Q -batch --no-build-details -eval '(write-region "" nil 
> "/tmp/test.txt")'
>
> /tmp/.#test.txt still exists.  This is probably because the function
> `current_lock_owner' doesn't handle the case where `system-name' returns
> nil (it assumes the file is remote in that case).

Previously reported (and ignored) in https://debbugs.gnu.org/34448