signal-logger: Don’t pass NULL to G_VALUE_LCOPY()

While it may have previously worked, it’s not how the API was meant to
be used, and a change in GLib 2.66 means that it now causes a critical
warning to be emitted (see
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1505#note_984868

).

Fix that by using a custom version of `G_VALUE_LCOPY()`.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>