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

This closes #2168
diff --git a/lib/py/src/ext/protocol.h b/lib/py/src/ext/protocol.h
index 521b7ee..c0cd437 100644
--- a/lib/py/src/ext/protocol.h
+++ b/lib/py/src/ext/protocol.h
@@ -35,7 +35,7 @@
   ProtocolBase()
     : stringLimit_((std::numeric_limits<int32_t>::max)()),
       containerLimit_((std::numeric_limits<int32_t>::max)()),
-      output_(NULL) {}
+      output_(nullptr) {}
   inline virtual ~ProtocolBase();
 
   bool prepareDecodeBufferFromTransport(PyObject* trans);