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/WorkoutGraph.cpp

@@ -64,7 +64,7 @@ void WorkoutGraph::SetInternal( int iMinSongsPlayed )
 	vector<int> viMeters;
 	for (TrailEntry const &e : pTrail->m_vEntries)
 	{
-		ASSERT( e.pSteps != NULL );
+		ASSERT( e.pSteps != nullptr );
 		viMeters.push_back( e.pSteps->GetMeter() );
 	}