fix compiler warnings in tools and test-

code/glTF2/glTF2Importer.cpp

@@ -4,7 +4,6 @@ Open Asset Import Library (assimp)
 
 Copyright (c) 2006-2020, assimp team
 
-
 All rights reserved.
 
 Redistribution and use of this software in source and binary forms,
@@ -96,7 +95,7 @@ glTF2Importer::glTF2Importer() :
 		BaseImporter(),
 		meshOffsets(),
 		embeddedTexIdxs(),
-		mScene(NULL) {
+		mScene(nullptr) {
 	// empty
 }