Fixed: [Activity] allow to set a user null

Fix https://gitlab.com/Chill-Projet/chill-bundles/-/issues/74

src/Bundle/ChillActivityBundle/Form/ActivityType.php

@@ -225,6 +225,7 @@ class ActivityType extends AbstractType
             $builder->add('user', PickUserDynamicType::class, [
                 'label' => $activityType->getLabel('user'),
                 'required' => $activityType->isRequired('user'),
+                'multiple' => false,
             ]);
         }