Issue #3611433: Fix addcslashes() null deprecation when validating an empty domain path alias (2.x backport)

Backport of !123 (merged) to the 2.x branch. The bug is present on 2.x: DomainPathUniqueConstraintValidator runs the uniqueness lookup with a NULL alias for items that have a domain but no alias, triggering addcslashes() deprecation. Same fix: early-return in the validator when the alias is empty, plus a discriminating kernel regression test. Based on current 2.x.