xdg-shell: make wlr_xdg_surface_from_resource reject NULL

Most resources must not be NULL. Make it so callers need to check for
NULL explicitly. This makes it clearer in the handlers code that the
NULL wl_resource case needs to be handled, and allows callers to make a
difference between a NULL wl_resource and an inert resource.

types/xdg_shell/wlr_xdg_surface.c

@@ -237,8 +237,10 @@ static void xdg_surface_handle_get_popup(struct wl_client *client,
 		struct wl_resource *positioner_resource) {
 	struct wlr_xdg_surface *xdg_surface =
 		wlr_xdg_surface_from_resource(resource);
-	struct wlr_xdg_surface *parent =
-		wlr_xdg_surface_from_resource(parent_resource);
+	struct wlr_xdg_surface *parent = NULL;
+	if (parent_resource != NULL) {
+		parent = wlr_xdg_surface_from_resource(parent_resource);
+	}
 	if (xdg_surface == NULL) {
 		return; // TODO: create an inert xdg_popup
 	}
@@ -517,9 +519,6 @@ void destroy_xdg_surface(struct wlr_xdg_surface *surface) {
 
 struct wlr_xdg_surface *wlr_xdg_surface_from_resource(
 		struct wl_resource *resource) {
-	if (!resource) {
-		return NULL;
-	}
 	assert(wl_resource_instance_of(resource, &xdg_surface_interface,
 		&xdg_surface_implementation));
 	return wl_resource_get_user_data(resource);