THRIFT-5225: Use nullptr instead of NULL
Patch: Zezeng Wang

This closes #2168
diff --git a/lib/cpp/test/TFileTransportTest.cpp b/lib/cpp/test/TFileTransportTest.cpp
index 21c1f3b..c116fdc 100644
--- a/lib/cpp/test/TFileTransportTest.cpp
+++ b/lib/cpp/test/TFileTransportTest.cpp
@@ -399,6 +399,6 @@
 
   // Parse arguments
   parse_args(argc, argv);
-  return NULL;
+  return nullptr;
 }
 #endif