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/channel_mgmt.cpp b/user_channel/channel_mgmt.cpp
index 0b76330..69958fa 100644
--- a/user_channel/channel_mgmt.cpp
+++ b/user_channel/channel_mgmt.cpp
@@ -952,7 +952,7 @@
             if (jsonChInfo[mediumTypeString].get<std::string>() == "lan-802.3")
             {
                 channelFound = false;
-                for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next)
+                for (ifa = ifaddr; ifa != nullptr; ifa = ifa->ifa_next)
                 {
                     if (jsonChData[nameString].get<std::string>() ==
                         ifa->ifa_name)