[BUGFIX release] Add support for null prototype object to mandatory setter

Objects created with Object.create(null) do not have the method
propertyIsEnumerable. Mandatory setter expects this.

Here is choose to presume any object not having the detection method
would return true. This is inline with the output of:

```
var nullObject = Object.create(null);
nullObject['foo'] = 'bar';
Object.prototype.propertyIsEnumerable.call(nullObject, 'foo');
// ^ is true
```

Fixes #9433 (applied cleanly to master, I expect to stable too)