runDialog: Don’t treat empty input specially

runDialog: Don’t treat empty input specially

Let empty input result in an error, just like other invalid commands
(bad syntax, nonempty whitespace, etc.). GLib already has an error
message “Text was empty (or contained only whitespace)” which it shows
for whitespace-only input, so letting that message apply also to empty
input makes sense.

This requires some tweaks further down the file to avoid interpreting
empty input as an empty path (relative to the home directory) and then
opening the home directory.

Part of https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3183.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1442>