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 GNOME/glib!1505 (comment 984868)).
Fix that by using a custom version of G_VALUE_LCOPY()
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Signed-off-by: Philip Withnall pwithnall@endlessos.org