[BUGFIX beta] Prevent mandatory-setter when setter is already present.

Closes #9899.
Fixes #9897.
> > we are working on a Object.create(null) object
>
> im not sure how this matters

You are correct, it does not change the `descriptor` values (I was wrong above), but it does make `obj.hasOwnProperty` not exist so we must do `keyName in obj` instead of `obj.hasOwnProperty(keyName)`.