preferences-window: Don't strip mnemonics of empty strings

Because there's no point in doing so.
3 jobs for preferences-window-string-fix in 1 minute and 44 seconds
Status Name Job ID Coverage
  Build
passed api-visibility #1229469

00:00:11

passed build-flatpak #1229471
flatpak

00:01:03

passed doc #1229470
flatpak

00:01:44