C++11: Replaced the C NULL with the safer C++11 nullptr

Requires gcc version 4.7 or higher

src/OpenFOAM/db/functionObjects/functionObject/functionObject.H

@@ -180,7 +180,7 @@ public:
         autoPtr<functionObject> clone() const
         {
             NotImplemented;
-            return autoPtr<functionObject>(NULL);
+            return autoPtr<functionObject>(nullptr);
         }