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.

Change-Id: I45ad10d46e3d01e66717bb47225aff8153f60c43
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/include/sdbusplus/utility/memory.hpp b/include/sdbusplus/utility/memory.hpp
index 11755be..a9d98e5 100644
--- a/include/sdbusplus/utility/memory.hpp
+++ b/include/sdbusplus/utility/memory.hpp
@@ -5,7 +5,7 @@
 static inline void* mfree(void* memory)
 {
     free(memory);
-    return NULL;
+    return nullptr;
 }
 
 static inline void freep(void* p)