fix fatal error on audio.play_sound in headless-mode

src/logic/scripting/lua/libs/libaudio.cpp

@@ -31,6 +31,9 @@ inline audio::speakerid_t play_sound(
         return 0;
     }
     auto assets = scripting::engine->getAssets();
+    if (assets == nullptr) {
+        return 0;
+    }
     auto sound = assets->get<audio::Sound>(name);
     if (sound == nullptr) {
         return 0;