[REV] Revert "[FIX] mail: avoid crashing for users without email address"

This reverts commit 6230f72d.

Crashing when user have no valid email is the expected behavior. Flows sending
emails must ensure either current user has valid login / email, either it
uses an user OR author_id / email_from that has a meaning in the functional
flow. Notably when public user is involved. Hiding functional issues is not
what we want.