ipc: handle unnamed xkb_active_layout_name

If the active xkb_layout does not have a name, use `NULL` instead of
`json_object_new_string(NULL)`. This also makes it so swaymsg will pretty
print this as `(unnamed)`.

sway/ipc-json.c

@@ -597,7 +597,7 @@ json_object *ipc_json_describe_input(struct sway_input_device *device) {
 				const char *layout =
 					xkb_keymap_layout_get_name(keymap, layout_idx);
 				json_object_object_add(object, "xkb_active_layout_name",
-						json_object_new_string(layout));
+						layout ? json_object_new_string(layout) : NULL);
 				break;
 			}
 		}