Fix retention rule validation in create/edit layout

Steps to reproduce:

• Go to the retention dawer menu and click Retention rules
• Create a new retention rule
• Fill required title field
• In RETENTION PERIOD STARTING POINT / Define when the retention period starts, select Based on metadata. Xpath required field appears -> leave it blank
• Then select Immediate
• Fill other required Years, Months, Days duration fields
• Click Create -> KO nothing happens

It is because the validation fails as the Xpath field is inside a dom-if template and is stamped when selection Based on metadata policy is selected. Once unselected, it remains stamped and is processed by the form validation. Indeed:

By default, stamped elements will be hidden but left in the DOM when if becomes false, which is generally results in better performance.

See https://polymer-library.polymer-project.org/3.0/api/elements/dom-if#DomIf-property-restamp