Empty groups on thesaurus

[Zasa-san]

Describe the bug When creating a new thesaurus, you are allowed to create an empty group.

To Reproduce Steps to reproduce the behavior:

1. Go to thesauri settings screen
2. Add thesaurus
3. Add group
4. Simply put a name to the group and hit Add
5. If there is no other items, it seems you can save the group, but then you cant edit it to add items. If the thesaurus has items, then it throws an error when trying to save.

Expected behavior Validate that groups cannot be created empty, and when deleting items in a group remove the whole group if it's left with no items.