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: Ia40d9435ab57651ec6d13d5408cf69130043fa79
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/groupext.cpp b/groupext.cpp
index 222ce9a..f61bf1f 100644
--- a/groupext.cpp
+++ b/groupext.cpp
@@ -24,7 +24,7 @@
 void registerNetFnGroupExtFunctions()
 {
     // <Group Extension Command>
-    ipmi_register_callback(NETFUN_GRPEXT, GRPEXT_GET_GROUP_CMD, NULL,
+    ipmi_register_callback(NETFUN_GRPEXT, GRPEXT_GET_GROUP_CMD, nullptr,
                            ipmi_groupext, PRIVILEGE_USER);
 
     return;