STYLE: use 'return nullptr' for empty autoPtr/tmp returns

- both autoPtr and tmp are defined with an implicit construct from
  nullptr (but with explicit construct from a pointer to null).
  Thus is it safe to use 'nullptr' when returning an empty autoPtr or tmp.

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

@@ -197,7 +197,7 @@ public:
         autoPtr<functionObject> clone() const
         {
             NotImplemented;
-            return autoPtr<functionObject>();
+            return nullptr;
         }