(bug 41337) Fix fatal error in Special:Preferences

The error is Fatal error: Call to a member function msg() on a
non-object at includes/Preferences.php on line 1207.

The problem is that fields created in Preferences::getPreferences() for
validation don't have their parent set and thus an error occurs when the
validation fails since they want to use their parent to get a Message
object.

This commit adds a dummy parent object to these fields to fix the error.

bug: 41337
Change-Id: I5826d6e3f1262c8d26af0cfe7074a939f80bcaca