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/ScreenOptionsManageEditSteps.cpp

@@ -140,7 +140,7 @@ void ScreenOptionsManageEditSteps::HandleScreenMessage( const ScreenMessage SM )
 		else	// a Steps
 		{
 			Steps *pSteps = GAMESTATE->m_pCurSteps[PLAYER_1];
-			ASSERT( pSteps != NULL );
+			ASSERT( pSteps != nullptr );
 			const Style *pStyle = GAMEMAN->GetEditorStyleForStepsType( pSteps->m_StepsType );
 			GAMESTATE->SetCurrentStyle( pStyle );
 			// do base behavior