Index column of child domain class should not be nullable

### Task List

- [X] Steps to reproduce provided
- [ ] Stacktrace (if present) provided
- [X] Example that reproduces the problem uploaded to Github
- [X] Full description of the issue provided (see below)

### Steps to Reproduce

1. Clone [this repo](https://github.com/donalmurtagh/grails-list-index-bug)
2. Create a MySQL database on localhost named `list_index` that can be accessed by the `root` user with a blank password
3. Start the application with `grails run-app`

### Expected Behaviour

The `images_idx` column of the `image` table should not be nullable, because each image should have an index

### Actual Behaviour

The `images_idx` column of the `image` table is nullable

### Environment Information

- **Operating System**: Mac OS X
- **GORM Version:** 6.1.9.RELEASE
- **Grails Version (if using Grails):** 3.3.5
- **JDK Version:** 1.8.0_66

### Example Application

- https://github.com/donalmurtagh/grails-list-index-bug

Start with the linked grails-list-index-bug example application and run it with `grails run-app` against the stated MySQL database. Inspect the generated `image` table and its `images_idx` column; done means the column is created as non-nullable while preserving the child domain class behavior.

Written by the indexing model from the issue text.