Issue #3592825 by mably: [3.0.x backport] Toggler link element has no #parents, causing Undefined array key #parents on validation error
Backport to 3.0.x of the fix merged to 3.x (commit 15468931).
The toggler link element had no #parents, causing an "Undefined array key #parents" warning on any validation error of a domain-enabled config form. Sets an explicit empty #parents on the display-only link.
Cherry-pick applied cleanly (the toggler link element is the same on 3.0.x). Verified locally on 3.0.x: DomainConfigUIPermissionsTest passes (2 tests, 46 assertions), including the added testTogglerParentsOnConfigTargetForm.