Fixes the issue where this returns all the documents:
```
{
name: 'post',
type: 'relationship',
relationTo: 'posts',
filterOptions: { id: { in: [] } }
}
```
The issue isn't with the Local API but with how we send the query to the
REST API through `qs.stringify`. `qs.stringify({ id: { in: [] } }`
becomes `""`, so the server ignores the original query. I don't think
it's possible to encode empty arrays with this library
https://github.com/sindresorhus/query-string/issues/231, so I just made
sanitization to `{ exists: false }` for this case.
8ebadd4190