fix(dropdownfield): prevent PHP notice

for glpi object fields / users without the entity restriction set; see commit 79af87d

inc/field/dropdownfield.class.php

@@ -254,6 +254,7 @@ public function buildParams($rand = null) {
             $dparams['right'] = 'all';
             $currentEntity = Session::getActiveEntity();
             $ancestorEntities = getAncestorsOf(Entity::getTable(), $currentEntity);
+            $decodedValues['entity_restrict'] = $decodedValues['entity_restrict'] ?? self::ENTITY_RESTRICT_FORM;
             switch ($decodedValues['entity_restrict']) {
                case self::ENTITY_RESTRICT_FORM:
                   $form = new PluginFormcreatorForm();