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: Ie9e20e9f97d9a2b974a4a8748a3daaf5d516adb8
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/test/bej_encoder_test.cpp b/test/bej_encoder_test.cpp
index f40706f..413d4e1 100644
--- a/test/bej_encoder_test.cpp
+++ b/test/bej_encoder_test.cpp
@@ -281,7 +281,7 @@
 
     for (int i = 0; i < linkCount; ++i)
     {
-        addLinkToTree(&linksInfo->array, &linkJsonArray[i].set, NULL,
+        addLinkToTree(&linksInfo->array, &linkJsonArray[i].set, nullptr,
                       &linkJsonArray[i].odataId, links[i]);
     }
 }
@@ -361,7 +361,7 @@
         .stackPeek = stackPeek,
         .stackPop = stackPop,
         .stackPush = stackPush,
-        .deleteStack = NULL,
+        .deleteStack = nullptr,
     };
 
     bejEncode(&dictionaries, BEJ_DICTIONARY_START_AT_HEAD, bejMajorSchemaClass,