Fix ffap behavior in the Gnus group buffer (bug#35693)

* lisp/ffap.el (ffap-file-exists-string, ffap-file-at-point):
Don't recognize "" as a path or a file name (bug#35693).
1 job for master in 88 minutes and 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #1786
test-all

01:28:04

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:957: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1