surface: fix abort on NULL buffer attach

Fixes: ba0525c5c0 ("surface: drop wlr_surface_state.buffer_resource")
Closes: https://github.com/swaywm/wlroots/issues/3173

types/wlr_surface.c

@@ -42,11 +42,14 @@ static void surface_handle_attach(struct wl_client *client,
 		struct wl_resource *buffer_resource, int32_t dx, int32_t dy) {
 	struct wlr_surface *surface = wlr_surface_from_resource(resource);
 
-	struct wlr_buffer *buffer = wlr_buffer_from_resource(buffer_resource);
+	struct wlr_buffer *buffer = NULL;
+	if (buffer_resource != NULL) {
+		buffer = wlr_buffer_from_resource(buffer_resource);
 		if (buffer == NULL) {
 			wl_resource_post_error(buffer_resource, 0, "unknown buffer type");
 			return;
 		}
+	}
 
 	surface->pending.committed |= WLR_SURFACE_STATE_BUFFER;
 	surface->pending.dx = dx;