Terms lose <root> as the parent when editing

Problem/Motivation

When editing a taxonomy term which has more than one parent, where one of the parents is root, root will be lost on save.

Steps to reproduce:

1. Create a term (term1).
2. Create another term (term2) which has the parents set to root and term1.
3. Edit term2, save without editing the parents.
4. Term2 will now only have one parent, term1. It would've lost root as a parent.

This is because when loading a terms parents it loads an array of terms, root is not a term so this doesn't get loaded.

Proposed resolution

Don't assume a parent is always a term entity.

Remaining tasks

- Reroll the latest patch based on 9.4.x.
- Manual testing after patch can be applied.
- Patch needss review

User interface changes

API changes

Data model changes