[iOS] Move logic to check for required empty fields from VC to mediator

The logic to check if either of the required field has been empty has
been moved from the view controller AutofillProfileEditTableViewController
to the mediator AutofillProfileEditMediator.

Bug: 1482269
Change-Id: I1d4e6911e15da5ed38bc4154363de36f279697a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5476410
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Vidhan Jain <vidhanj@google.com>
Cr-Commit-Position: refs/heads/main@{#1291790}