Don't set slug explicitely to null in wp_update_nav_menu_object

[Chouby]

This new ticket is related to #23199, #21950, #5809 (shared terms)

If a user creates a new nav menu with the same name as an existing term in another taxonomy, the term is shared with this other taxonomy. This is known and probably not an issue here.

But when the menu is saved, calling update_term with the slug explicitely set to null does modify the (shared) term slug.

[nacin]

Makes sense at a glance. Unit tests would help make this happen.

[Chouby]

Obviously, this ticket should have been fixed by the resolution of #5809. I propose to close it.