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/postProcessing/lumped/lumpedPointForces/lumpedPointForces.C

@@ -71,7 +71,7 @@ autoPtr<GeoFieldType> loadField
         );
     }
 
-    return autoPtr<GeoFieldType>();
+    return nullptr;
 }