Hide the 'Add existing [label]' form / button if there are none

This doesn't depend on the other issue at all, we could commit this here any time.

I don't love having to add the extra layer of indentation, but the code you inherited was already a bit of a mess, so it's not so easy to do it otherwise without a larger code refactoring.

The patch did not apply, but i think i brought this in faithfully except the ief_items and ief_add_existing_widget properties which did not exist in this module and do not yet exist in Inline Entity Form either. Talking about these lines that existed in the patch both as if already there and only getting indented in for the conditional:

          '#ief_items' => $items,
          '#ief_add_existing_widget' => $this->getSetting('add_existing_widget'),

OK i see those are from ✨ Support the 'add_existing_widget' setting from #2683125 Needs review as i thought and should have already checked. Still willing to adopt everything that is stuck getting into IEF and then, i hope, getting everything into IEF (in particular ✨ Prioritize 'add existing' in nested entity creation UX to prevent duplication. Needs work ) and obsoleting this module entirely!