ART: Replace NULL to nullptr in the optimizing compiler

Replace macro NULL to the nullptr variation for C++.

Change-Id: Ib6e48dd4bb3c254343383011b67372622578ca76
Signed-off-by: Jean Christophe Beyler <jean.christophe.beyler@intel.com>
diff --git a/compiler/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index fb941b5..bcd3f0a 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -445,7 +445,7 @@
   HUseListNode<T>* current = *list;
   while (current != nullptr) {
     if (current->GetUser() == user && current->GetIndex() == input_index) {
-      if (previous == NULL) {
+      if (previous == nullptr) {
         *list = current->GetTail();
       } else {
         previous->SetTail(current->GetTail());