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/functionObjects/solvers/energyTransport/energyTransport.C

@@ -105,7 +105,8 @@ Foam::functionObjects::energyTransport::kappaEff() const
 
     FatalErrorInFunction
         << "Turbulence model not found" << exit(FatalError);
-    return tmp<volScalarField>();
+
+    return nullptr;
 }