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

Fixes the deprecation "addcslashes(): Passing null to parameter #1 ($string) of type string is deprecated" raised when saving an entity whose domain path item has a domain assigned but an empty alias.

The DomainPathUniqueConstraintValidator now returns early when the alias is empty: an empty alias cannot collide with an existing one, and skipping the lookup avoids passing an empty value to the database escape function.

Adds a discriminating kernel regression test (asserts no uniqueness query runs for an empty alias) that fails on the unpatched code and passes with the fix. Branch is based on current 3.x.