output: fix crash in output_test_with_back_buffer()

The output->impl->test check has been removed, but
output_test_with_back_buffer() hasn't been updated accordingly.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3506
Fixes: 3be6658ee7 ("output: allocate swapchain on first commit")

types/output/render.c

@@ -166,7 +166,9 @@ static bool output_attach_empty_back_buffer(struct wlr_output *output,
 
 static bool output_test_with_back_buffer(struct wlr_output *output,
 		const struct wlr_output_state *state) {
-	assert(output->impl->test != NULL);
+	if (output->impl->test == NULL) {
+		return true;
+	}
 
 	// Create a shallow copy of the state with the empty back buffer included
 	// to pass to the backend.