The big NULL replacement party part 1.

This is meant to be a safer alternative since
NULL can often be 0. Let's not rely on that.

And yes, I know this is a lot of files. This is
a safer thing to do in big commits vs for loops.

src/Background.cpp

@@ -373,7 +373,7 @@ bool BackgroundImpl::Layer::CreateBackground( const Song *pSong, const Backgroun
 
 	Actor *pActor = ActorUtil::MakeActor( sEffectFile );
 
-	ASSERT( pActor != NULL );
+	ASSERT( pActor != nullptr );
 	m_BGAnimations[bd] = pActor;
 
 	for( unsigned i=0; i<vsResolvedRef.size(); i++ )