InfluxDB 2: List all labels for a template is null instead of empty array

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The list of all labels for a template should be empty array instead of null if there is no one.

```bash
# Create template
--> POST http://127.0.0.1:9999/api/v2/documents/templates
--> {"meta":{"name":"document-name1584085373830-IT","version":"1"},"content":"templates content","orgID":"8fddb6e6c18dd499","labels":[]}
<-- 201 Created http://127.0.0.1:9999/api/v2/documents/templates
<-- {
"links": {
"self": "/api/v2/documents/templates/0560b57dea990000"
},
"id": "0560b57dea990000",
"meta": {
"name": "document-name1584085373830-IT",
"version": "1",
"createdAt": "2020-03-13T07:42:53.8662117Z",
"updatedAt": "2020-03-13T07:42:53.8662266Z"
},
"content": "templates content"
}

### List all labels
--> GET http://127.0.0.1:9999/api/v2/documents/templates/0560b57dea990000/labels
<-- 200 OK http://127.0.0.1:9999/api/v2/documents/templates/0560b57dea990000/labels
<-- {"links":{"self":"/api/v2/labels"},"labels":null}
```

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Create new Template
2. List labels for template

__Expected behavior:__
```
{"links":{"self":"/api/v2/labels"},"labels":[]}
```

__Actual behavior:__
```
{"links":{"self":"/api/v2/labels"},"labels":null}
```

__Environment info:__

* InfluxDB version: docker v2.0.0-beta.6, latest nightly build

Start by reproducing the issue with POST /api/v2/documents/templates using an empty labels array, then GET /api/v2/documents/templates/{id}/labels. Trace the API entry point that builds the labels response. Done means the endpoint returns "labels":[] when the template has no labels, while preserving the documented links and status response.

Written by the indexing model from the issue text.