Right now, when some declarative configuration having empty tags
on any of its entities is deployed, subsequent `diff` / `sync`
will show misleading results:

```
$ cat kong.yaml
_format_version: "3.0"
services:
  - name: foo
    protocol: http
    host: foo
    port: 8000
    tags: []

$ deck sync
creating service foo
Summary:
  Created: 1
  Updated: 0
  Deleted: 0

$ deck sync
updating service foo  {
   "connect_timeout": 60000,
   "enabled": true,
   "host": "foo",
   "id": "afadd2ed-2bcd-49fc-9000-adb58cd11b50",
   "name": "foo",
   "port": 8000,
   "protocol": "http",
   "read_timeout": 60000,
   "retries": 5,
   "write_timeout": 60000
 }

Summary:
  Created: 0
  Updated: 1
  Deleted: 0
```

The second `deck sync` run is showing an update without an actual diff.
This is due to the way empty tags are treated internally in decK.

This commit fixes this defect.