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/ScreenGameplayLesson.cpp

@@ -15,8 +15,8 @@ ScreenGameplayLesson::ScreenGameplayLesson()
 
 void ScreenGameplayLesson::Init()
 {
-	ASSERT( GAMESTATE->GetCurrentStyle() != NULL );
-	ASSERT( GAMESTATE->m_pCurSong != NULL );
+	ASSERT( GAMESTATE->GetCurrentStyle() != nullptr );
+	ASSERT( GAMESTATE->m_pCurSong != nullptr );
 
 	/* Now that we've set up, init the base class. */
 	ScreenGameplayNormal::Init();