Replace NULL macros with nullptr literals.

Change-Id: I918c40879aa547438f77e7d1a95fa2aa33bec398
diff --git a/rsMesh.h b/rsMesh.h
index 9b61ebe..c7ee088 100644
--- a/rsMesh.h
+++ b/rsMesh.h
@@ -45,7 +45,7 @@
             Allocation **vertexBuffers;
             uint32_t vertexBuffersCount;
 
-            // indexBuffers[i] could be NULL, in which case only primitives[i] is used
+            // indexBuffers[i] could be nullptr, in which case only primitives[i] is used
             Allocation **indexBuffers;
             uint32_t indexBuffersCount;
             RsPrimitive *primitives;