Use "Unknown user" instead of an empty user name.

Use "Unknown user" instead of an empty user name.

This changes the user name to "User unknown" when constructing a RevisionRecord
from a database row that has an empty ar_user_text resp rev_user_text field.

This may cause "User unknown" to be written to the database, if the
RevisionRecord is used as the basis for a new revision that is being created,
particularly during undeletion. Since "Unknown user" is listed in
$wgReservedUsernames, this should never lead to conflicts with actual user
names.

It is assumed that empty ar_user_text and rev_user_text fields will be
fixed during migration to the new actor based database schema.

Bug: T195692
Change-Id: I506c513b019778d83741e47f0d11093f5ab67a54