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/BGAnimation.cpp

@@ -49,7 +49,7 @@ void BGAnimation::AddLayersFromAniDir( const RString &_sAniDir, const XNode *pNo
 		for (RString const &sLayer : vsLayerNames)
 		{
 			const XNode* pKey = pNode->GetChild( sLayer );
-			ASSERT( pKey != NULL );
+			ASSERT( pKey != nullptr );
 
 			RString sImportDir;
 			if( pKey->GetAttrValue("Import", sImportDir) )