fix: materialized path being computed as "undefined1."

[mschwebler-tgm]

Description of change

When tree entities are saved implicitly (through another related entity which hasMany tree-entities and "cascade" set to true) the ORM generates materialized-path strings prefixed with undefined. Resulting in materialized paths like undefined1.2.3. This pull request fixes that. Root cause was that the materialized path always relies on the previously inserted entity mpath which is undefined for the root-tree-entity.

Pull-Request Checklist

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

[pleerock]

please remove .only to make sure all tests are passing

[pleerock]

unfortunately Oracle tests are failing

[pleerock]

Thank you for contribution! 🎉