If 'Parent Link' field is updated then the Issue Updated event is generated, users get an email, and listeners can see the change, but when the field is cleared, i.e. empty value is set, then no event is generated, and listeners do no see the change.

Listeners (especially those provided by Scriptrunner) must be able to see the change when Parent Link is removed.

Steps to reproduce

1. Set a Parent link value in an issue
2. Check issue history tab to see the changes to Parent Link field being reflected there
3. Search with a JQL like "Parent Link" = '<Issue Key>' and see that the issue can be searched correctly
4. Now remove the value of Parent link on the issue
5. Navigate to the history tab to see the changes to Parent Link field NOT being reflected there. There is no entry in there for this action.
6. Try to search for the issue using "Parent Link" is EMPTY. The issue is not searchable

Expected behaviour

Parent Link field should behave like any other custom field and log any changes made to the value of the Parent link field.