fix errors to user picker: add placeholder and allow empty

Validator/Constraints/Entity/UserCircleConsistencyValidator.php

@@ -50,6 +50,10 @@ class UserCircleConsistencyValidator extends ConstraintValidator
         /* @var $user \Chill\MainBundle\Entity\User */
         $user = \call_user_func([$value, $constraint->getUserFunction ]);
         
+        if ($user === null) {
+            return;
+        }
+        
         if (FALSE === $this->autorizationHelper->userHasAccess($user, $value, $constraint->role)) {
             $this->context
                 ->buildViolation($constraint->message)