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.

applications/utilities/parallelProcessing/reconstructParMesh/reconstructParMesh.C

@@ -251,7 +251,7 @@ autoPtr<mapPolyMesh> mergeSharedPoints
 
     if (returnReduce(pointToMaster.size(), sumOp<label>()) == 0)
     {
-        return autoPtr<mapPolyMesh>();
+        return nullptr;
     }
 
     polyTopoChange meshMod(mesh);