emails: fix sending emails to invalid addresses.

Fix mailers to not attempt deliveries to invalid or nonexistent email
addresses. This usually happened when someone changed their email, and
we tried to send a confirmation email to a nonexistent address.

test/functional/users_controller_test.rb

@@ -259,7 +259,7 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
         assert_equal(User::Levels::MEMBER, User.last.level)
         assert_equal(User.last, User.last.authenticate_password("xxxxx1"))
         assert_nil(User.last.email_address)
-        assert_no_enqueued_emails
+        assert_enqueued_email_with UserMailer, :welcome_user, args: [User.last], queue: "default"
         assert_equal(true, User.last.user_events.user_creation.exists?)
       end