Replace NULL macros with nullptr literals.

Change-Id: I918c40879aa547438f77e7d1a95fa2aa33bec398
diff --git a/rsProgramStore.cpp b/rsProgramStore.cpp
index b2d527e..b07f820 100644
--- a/rsProgramStore.cpp
+++ b/rsProgramStore.cpp
@@ -71,7 +71,7 @@
 }
 
 ProgramStore *ProgramStore::createFromStream(Context *rsc, IStream *stream) {
-    return NULL;
+    return nullptr;
 }
 
 void ProgramStore::init() {