erp5_xhtml_style: fix json parsing data-gadget-renderjs-extra

When data-gadget-renderjs-extra attribute is not set to an element,
`element.getAttribute("data-gadget-renderjs-extra")` returns null.

Possible error messages :
- sub_key is null
- TypeError: Cannot set property 'key' of null

/reviewed-on nexedi/erp5!602

product/ERP5/bootstrap/erp5_xhtml_style/SkinTemplateItem/portal_skins/erp5_xhtml_style/erp5_gadgetfield.js.js

@@ -118,7 +118,7 @@
                 editable: (field_element.getAttribute("data-gadget-editable") !== null),
                 key: field_element.getAttribute("data-gadget-editable"),
                 value: field_element.getAttribute("data-gadget-value"),
-                extra: field_element.getAttribute("data-gadget-renderjs-extra")
+                extra: field_element.getAttribute("data-gadget-renderjs-extra") || "{}"
               });
               promise_list.push(gadget.getDeclaredGadget(field_scope));
             }