[data-list] don't show `key` input field when value is an array

If a list field is initialized with an array, such as `['foo', 'bar', 'baz']` then the `key` becomes the index of the item in the array:

```
0: foo
1: bar
2: baz
```

Currently the list field will display a `key` input, but the value ends up being ignored when the form is submitted.  It would be better to not allow input for the keys in this situation. 
