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/user_channel/user_mgmt.cpp b/user_channel/user_mgmt.cpp
index 3d9a82f..b5680e0 100644
--- a/user_channel/user_mgmt.cpp
+++ b/user_channel/user_mgmt.cpp
@@ -684,7 +684,7 @@
 {
     const struct pam_conv localConversation = {pamFunctionConversation,
                                                const_cast<char*>(password)};
-    pam_handle_t* localAuthHandle = NULL; // this gets set by pam_start
+    pam_handle_t* localAuthHandle = nullptr; // this gets set by pam_start
 
     int retval =
         pam_start("passwd", username, &localConversation, &localAuthHandle);
@@ -710,7 +710,7 @@
     const struct pam_conv localConversation = {
         pamFunctionConversation, const_cast<char*>(password.data())};
 
-    pam_handle_t* localAuthHandle = NULL; // this gets set by pam_start
+    pam_handle_t* localAuthHandle = nullptr; // this gets set by pam_start
 
     if (pam_start("dropbear", username.data(), &localConversation,
                   &localAuthHandle) != PAM_SUCCESS)