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

This closes #2168
diff --git a/contrib/fb303/cpp/ServiceTracker.h b/contrib/fb303/cpp/ServiceTracker.h
index 9a3edd8..faaa4a7 100644
--- a/contrib/fb303/cpp/ServiceTracker.h
+++ b/contrib/fb303/cpp/ServiceTracker.h
@@ -120,7 +120,7 @@
 public:
   enum Unit { UNIT_SECONDS, UNIT_MILLISECONDS, UNIT_MICROSECONDS };
   Stopwatch();
-  uint64_t elapsedUnits(Unit unit, std::string *label = NULL) const;
+  uint64_t elapsedUnits(Unit unit, std::string *label = nullptr) const;
   void reset();
 private:
   timeval startTime_;