Constraints on 'target_id' / 'tid' properties break autocomplete if applied

Context

Spin-off from #1992138: Helper issue for "field types as TypedData Plugins" / #1969728: Implement Field API "field types" as TypedData Plugins.

That issue adds actual validation on entity fields.
This means that implicit or explicit constraints contained in FieldItem::getPropertyDefinitions() (e.g 'type' => 'uri' in LinkItem, 'type' => 'integer' for 'tid' in TaxonomyTermReferenceItem...) that were just "dormant" so far in HEAD, are now fired.
This shows a couple problems, I'll open separate issues for them.

Issue

The "auto create / freetagging" feature of the autocomplete widgets (for entity_reference or taxo fields) break the constraint on 'target_id' / 'tid' being of type 'integer', because the items created by the "auto create" flow end up with a 'target_id' / 'tid' = FALSE, which is not an integer.

Patches coming up.