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

This closes #2168
diff --git a/lib/cpp/src/thrift/server/TNonblockingServer.cpp b/lib/cpp/src/thrift/server/TNonblockingServer.cpp
index 26ffa68..ea394c8 100644
--- a/lib/cpp/src/thrift/server/TNonblockingServer.cpp
+++ b/lib/cpp/src/thrift/server/TNonblockingServer.cpp
@@ -1341,7 +1341,7 @@
     FD_ZERO(&efds);
     FD_SET(fd, &wfds);
     FD_SET(fd, &efds);
-    ret = select(static_cast<int>(fd + 1), NULL, &wfds, &efds, NULL);
+    ret = select(static_cast<int>(fd + 1), nullptr, &wfds, &efds, nullptr);
     if (ret < 0) {
       return false;
     } else if (ret == 0) {