Allow list block with fields

It'd be great to allow an empty object, so that blocks that don't require an input are still able to be added (for section-drive content management).

```
label: 'Blocks'
name: 'blocks'
widget: 'list'
types:
- label: 'Hero'
name: 'hero'
widget: object
fields:
- { label: 'Title', name: 'title', widget: 'string' }
- { label: 'Sidebar Text', name: 'sidebar', widget: 'string', required: false }
- { label: 'Show Simplified Suffix', name: 'simplified', widget: 'boolean', required: false }
- label: 'Insights'
name: 'insights'
widget: object
```