allow short-circuiting lights logic when lights are not in use

stepmania/src/LightsManager.cpp

@@ -54,6 +54,9 @@ LightsManager::~LightsManager()
 
 void LightsManager::Update( float fDeltaTime )
 {
+	if( !IsEnabled() )
+		return;
+
 	// update lights falloff
 	{
 		FOREACH_CabinetLight( cl )

stepmania/src/LightsManager.h

@@ -53,6 +53,7 @@ public:
 	~LightsManager();
 	
 	void Update( float fDeltaTime );
+	bool IsEnabled() const { return m_pDriver != NULL; }
 
 	void BlinkCabinetLight( CabinetLight cl );
 	void BlinkGameButton( GameInput gi );

stepmania/src/arch/Lights/LightsDriver.cpp

@@ -19,12 +19,9 @@ LightsDriver *MakeLightsDriver(CString driver)
 	if( !driver.CompareNoCase("WeedTech") ) ret = new LightsDriver_LinuxWeedTech;
 	if( !driver.CompareNoCase("LinuxParallel") ) ret = new LightsDriver_LinuxParallel;
 #endif
-	if( !driver.CompareNoCase("Null") || ret == NULL )
-	{
-		if( driver.CompareNoCase("Null") )
-			LOG->Warn( "Unknown lights driver name: %s", driver.c_str() );
-		ret = new LightsDriver_Null;
-	}
+
+	if( ret == NULL && driver.CompareNoCase("Null") )
+		LOG->Warn( "Unknown lights driver name: %s", driver.c_str() );
 	
 	return ret;
 }