check if domain exists only if user account has a domain

app/models/concerns/user_username.rb

@@ -142,7 +142,7 @@ module UserUsername
   def validate_username
     if username_changed?
       # update_email if self.respond_to?(:email) TODO: if and only if email is linked to username
-      if defined?(Domain) && Domain.find_by(domain: self.domain).nil?
+      if defined?(Domain) && self.domain.present? && Domain.find_by(domain: self.domain).nil?
         errors.add :base, I18n.t('domain_invalid')
       end
       if User.username_taken?(self.username, self.domain) && errors[:username].empty?