Fix email with empty domain name labels passing validation (#23246)

* Fix email with empty domain name labels passing validation

`EmailMxValidator` would allow empty labels because `Resolv::DNS` is
particularly lenient about them, but the email would be invalid and
unusable.

* Add tests

app/validators/email_mx_validator.rb

@@ -10,6 +10,8 @@ class EmailMxValidator < ActiveModel::Validator
 
     if domain.blank?
       user.errors.add(:email, :invalid)
+    elsif domain.include?('..')
+      user.errors.add(:email, :invalid)
     elsif !on_allowlist?(domain)
       resolved_ips, resolved_domains = resolve_mx(domain)