UPBGE: Avoid to spawn sound actuator when no sound is loaded

This is to avoid a crash reported several times internally.

source/gameengine/Ketsji/KX_SoundActuator.cpp

@@ -177,7 +177,7 @@ void KX_SoundActuator::ProcessReplica()
 	SCA_IActuator::ProcessReplica();
 #ifdef WITH_AUDASPACE
 	m_handle = nullptr;
-	m_sound = AUD_Sound_copy(m_sound);
+	m_sound = m_sound ? AUD_Sound_copy(m_sound) : nullptr;
 #endif  // WITH_AUDASPACE
 }