Improve Workbench integration logic and also allow NULL title value if it is not a required field

Problem/Motivation

I was used to get this working correct for adding Title fields to nodes and making this field not required. After the patch from #2286145: Prevent empty translations from being synced into the legacy field it does not work anymore for this intended use.

Steps to reproduce:

Replace title by the Title field
Make this field NOT required
Add a new node with empty title (this works fine)
Edit or add a new node adding a value for Title (this works fine)
Edit a node with existing value for Title, making it now empty - this does not work

The expected outcome would be the node with no title.
What we actually see is the old value saved.

Proposed resolution

The bug comes from the logic added at #2286145: Prevent empty translations from being synced into the legacy field
We need to improve this logic to work in both scenarios, Workbench and empty title value.

Remaining tasks

User interface changes

API changes

Data model changes