Issue #3600795: Clear target_revision_id when the ERR entity is set to NULL

Mirrors the existing target_id null-handling for target_revision_id, so $item->set('entity', NULL) clears both target properties instead of fataling on getRevisionId(). Adds EntityReferenceRevisionsSaveTest::testSetNullEntityClearsTargetProperties. Follow-up to #2706883; the core/DefaultContent side is #3595546. Prepared with AI assistance.