Dont try and fullscreen a workspace with no views

Found this completely by accident

sway/commands.c

@@ -1435,6 +1435,9 @@ static struct cmd_results *cmd_fullscreen(int argc, char **argv) {
 		return error;
 	}
 	swayc_t *container = get_focused_view(&root_container);
+	if(container->type != C_VIEW){
+		return cmd_results_new(CMD_INVALID, "fullscreen", "Only views can fullscreen");
+	}
 	swayc_t *workspace = swayc_parent_by_type(container, C_WORKSPACE);
 	bool current = swayc_is_fullscreen(container);
 	wlc_view_set_state(container->handle, WLC_BIT_FULLSCREEN, !current);