backend/drm: fix return value type in create_drm_connector()

../backend/drm/drm.c:1435:24: error: incompatible types when returning type ‘_Bool’ but ‘struct wlr_drm_connector *’ was expected
     1435 |                 return false;
          |                        ^~~~~

backend/drm/drm.c

@@ -1408,7 +1408,7 @@ static struct wlr_drm_connector *create_drm_connector(struct wlr_drm_backend *dr
 
 	if (!get_drm_connector_props(drm->fd, wlr_conn->id, &wlr_conn->props)) {
 		free(wlr_conn);
-		return false;
+		return NULL;
 	}
 
 	const char *conn_name =