Cascading selection not respecting 'if none were selected'

Problem/Motivation

The "if none were selected" part of "On parent selection, automatically select children if none were selected.". Likewise for the reverse.

Steps to reproduce

- Create a taxonomy vocabulary with hierarchy
- Add a term field to a content type set to this vocab
- Set the form display widget to 'Term reference tree'
- Set 'Cascading selection' to anything but none
- Create some content of this type
- Select a child term in the field then select the parent. This will select all the other children, whereas from the wording I would expect no other children to be selected as I had already selected one individually

Proposed resolution

Updating the js to account for this.

Remaining tasks

Patch to follow shortly. Needs review.