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/file.hpp b/user_channel/file.hpp
index 204cfef..5be3a18 100644
--- a/user_channel/file.hpp
+++ b/user_channel/file.hpp
@@ -18,7 +18,7 @@
 {
   private:
     /** @brief handler for operating on file */
-    FILE* fp = NULL;
+    FILE* fp = nullptr;
 
     /** @brief File name. Needed in the case where the temp
      *         needs to be removed