use empty slice instead of nil slice when no tags exist for /v1/catalog/services

the docs for [/v1/catalog/services](http://www.consul.io/docs/agent/http.html#toc_22) show that the result should be like:

```json
{
    "consul": [],
    "redis": [],
    "postgresql": [
        "master",
        "slave"
    ]
}
```

but it was actually like:

```json
{
    "consul": null,
    "redis": null,
    "postgresql": [
        "master",
        "slave"
    ]
}
```

however, other endpoints (/v1/catalog/service/&lt;service&gt; and /v1/catalog/node/&lt;node&gt;) are documented to and actually return nulls for empty tags.  for consistency, maybe it makes more sense to update the docs for /v1/catalog/services to reflect the current behavior?