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/primitives/functions/Function1/Function1/Function1.C

@@ -79,7 +79,7 @@ Foam::tmp<Foam::Field<Type>> Foam::Function1<Type>::value
 ) const
 {
     NotImplemented;
-    return tmp<Field<Type>>();
+    return nullptr;
 }
 
 
@@ -100,7 +100,7 @@ Foam::tmp<Foam::Field<Type>> Foam::Function1<Type>::integrate
 ) const
 {
     NotImplemented;
-    return tmp<Field<Type>>();
+    return nullptr;
 }