replace NULL by nullptr on loadFile.

code/LWS/LWSLoader.cpp

@@ -501,7 +501,7 @@ void LWSImporter::InternReadFile(const std::string &pFile, aiScene *pScene,
     std::unique_ptr<IOStream> file(pIOHandler->Open(pFile, "rb"));
 
     // Check whether we can read from the file
-    if (file.get() == NULL) {
+    if (file.get() == nullptr) {
         throw DeadlyImportError("Failed to open LWS file " + pFile + ".");
     }