clang-tidy: Replace NULL with nullptr

Replaced all instances of NULL with nullptr to improve type safety
and clarity, as nullptr is the modern C++ standard for null pointers.

Tested: Build verified

Change-Id: If9e6c34c48821a7cf8577a2166727ce7db06fadc
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/oem/ibm/libpldmresponder/utils.cpp b/oem/ibm/libpldmresponder/utils.cpp
index 6eb26a5..393faab 100644
--- a/oem/ibm/libpldmresponder/utils.cpp
+++ b/oem/ibm/libpldmresponder/utils.cpp
@@ -72,7 +72,7 @@
     int nfd = sock + 1;
     int fd = -1;
 
-    int retval = select(nfd, &rfd, NULL, NULL, &tv);
+    int retval = select(nfd, &rfd, nullptr, nullptr, &tv);
     if (retval < 0)
     {
         error(
@@ -84,7 +84,7 @@
 
     if ((retval > 0) && (FD_ISSET(sock, &rfd)))
     {
-        fd = accept(sock, NULL, NULL);
+        fd = accept(sock, nullptr, nullptr);
         if (fd < 0)
         {
             error(
@@ -114,7 +114,7 @@
         FD_SET(sock, &wfd);
         int nfd = sock + 1;
 
-        int retval = select(nfd, NULL, &wfd, NULL, &tv);
+        int retval = select(nfd, nullptr, &wfd, nullptr, &tv);
         if (retval < 0)
         {
             error(