Fix add instructeur with empty mail

As select2 does not handle required attribute, we can only add server side logic

app/controllers/new_administrateur/groupe_instructeurs_controller.rb

@@ -49,7 +49,8 @@ module NewAdministrateur
     end
 
     def add_instructeur
-      emails = params['emails'].map(&:strip).map(&:downcase)
+      emails = params['emails'].presence || []
+      emails = emails.map(&:strip).map(&:downcase)
 
       correct_emails, bad_emails = emails
         .partition { |email| URI::MailTo::EMAIL_REGEXP.match?(email) }