fix(forms): null twig template value in item condition handler

Author: ccailly

templates/pages/admin/form/condition_handler_templates/item_dropdown.html.twig

@@ -49,7 +49,7 @@
 {{ fields.dropdownField(
     itemtype,
     input_name ~ '[items_id]',
-    input_value.items_id,
+    input_value.items_id|default(0),
     '',
     {
         'no_label'     : true,