The preview API doesn't apply the correct facets if the object has empty arrays

e.g. definition
https://dev.clearlydefined.io/definitions/git/github/automattic/mongoose/1ead0e616ab028a994ab47a23643749659243e07

add 2 globs (e.g.`**/docs/*`) to the **dev** and **data** facets. This will make the following API call:

```
https://dev-api.clearlydefined.io/definitions/git/github/automattic/mongoose/1ead0e616ab028a994ab47a23643749659243e07?preview=true

{"described":{"facets":{"data":["**/docs/*"],"dev":["**/docs/*"]}}}
```

Remove the blob on the **dev** facet and the body for the API call will be:

```json
{"described":{"facets":{"data":["**/docs/*"],"dev":[]}}}
```

In the response none of the files under the _docs_ folder have either facet applied to it