Metadata editor - parent identifier element doesn't display the Add button when missing

According to this configuration if the `gmd:parentIdentifier` element is missing should be displayed an `Add parent record` button, but at least in 3.8.x doesn't work:

https://github.com/geonetwork/core-geonetwork/blob/5b9585fb5522b05b2daed079b05639ceb871c199/schemas/iso19139/src/main/plugin/iso19139/layout/config-editor.xml#L3160-L3161

Test:

1) Create an iso19139 from default template
2) In metadata section at the bottom, no button to add the parent identifier.
3) Add it in the resources panel --> The metadata section displays the field now
4) Remove it --> The button is displayed for a moment, but when reloaded the editor the button and the field are not displayed anymore

Start with schemas/iso19139/src/main/plugin/iso19139/layout/config-editor.xml at lines 3160-3161 and reproduce the four metadata-editor steps using an iso19139 default template. Trace why the Add parent record control disappears after reload; done means the button and field behavior matches the configuration when parentIdentifier is absent.

Written by the indexing model from the issue text.