fix: find descendants of a non-existing tree parent

[astritsh]

Closes: #8556

Description of change

If subquery in findDescendants CONCAT(${subQuery.getQuery()}, '%') returns null when entity parent does not exist, the search string becomes LIKE '%', because CONCAT ignores NULL. This LIKE '%' results on returning everything instead of nothing.
Now with NULLIF fix, search will be LIKE NULL, which returns nothing for a non existing tree parent.
In case of Sqlite, LIKE ${subQuery.getQuery()} || '%' is already evaluated as LIKE NULL when subquery returns null.

Pull-Request Checklist

• [x ] Code is up-to-date with the master branch
• [ x] npm run lint passes with this change
• [ x] npm run test passes with this change
• [ x] This pull request links relevant issues as Fixes #0000
• [x ] There are new or updated unit tests validating the change
• Documentation has been updated to reflect this change
• [ x] The new commits follow conventions explained in CONTRIBUTING.md

[pleerock]

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