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: If9e6c34c48821a7cf8577a2166727ce7db06fadc
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/libpldmresponder/test/libpldmresponder_fru_test.cpp b/libpldmresponder/test/libpldmresponder_fru_test.cpp
index f4a1202..8eba53d 100644
--- a/libpldmresponder/test/libpldmresponder_fru_test.cpp
+++ b/libpldmresponder/test/libpldmresponder_fru_test.cpp
@@ -136,21 +136,21 @@
 
     pldm_entity_node* node =
         pldm_entity_association_tree_find(entityTree.get(), &systemEntity);
-    EXPECT_TRUE(node != NULL);
+    EXPECT_TRUE(node != nullptr);
 
     node = pldm_entity_association_tree_find(entityTree.get(), &chassisEntity);
-    ASSERT_TRUE(node != NULL);
+    ASSERT_TRUE(node != nullptr);
     test_pldm_entity_node* test_node = (test_pldm_entity_node*)node;
     EXPECT_TRUE((test_node->parent).entity_type == systemEntity.entity_type);
 
     node =
         pldm_entity_association_tree_find(entityTree.get(), &motherboardEntity);
-    ASSERT_TRUE(node != NULL);
+    ASSERT_TRUE(node != nullptr);
     test_node = (test_pldm_entity_node*)node;
     EXPECT_TRUE((test_node->parent).entity_type == chassisEntity.entity_type);
 
     node = pldm_entity_association_tree_find(entityTree.get(), &panelEntity);
-    EXPECT_TRUE(node == NULL);
+    EXPECT_TRUE(node == nullptr);
 }
 
 TEST(FruImpl, entityByObjectPath)