fix errors to user picker: add placeholder and allow empty

Form/Type/UserPickerType.php

@@ -83,7 +83,7 @@ class UserPickerType extends AbstractType
 
         $resolver
             ->setDefault('class', User::class)
-            ->setDefault('empty_data', $this->tokenStorage->getToken()->getUser())
+            ->setDefault('placeholder', 'Choose an user')
             ->setDefault('choice_label', function(User $u) {
                 return $u->getUsername();
             })