Generate valid JSON if partition table is empty

Generate valid JSON if partition table is empty

When the partition table is present **but** empty, the existing code would output fields followed by a `,` , on the assumption that the list of partitions would follow. But if the list of partitions is empty, it is skipped, leading to output like this:

```
{
  "partitiontable": {
      "label":"gpt",
      "id":"1F9E80D9-DD78-024F-94A3-B61EC82B18C8",
      "device":"/dev/sdb",
      "unit":"sectors",
      "firstlba":2048,
      "lastlba":30949342,
      "sectorsize":512,
  }
}
```

Note the `512,` on the *sectorsize* line.

This is invalid JSON for some parsers, which choke on it.

Avoid this, by checking when outputting the last separator: if there's no table, or there is a table but it is empty, then just put a newline, otherwise use the old path of comma-newline and assume there's going to be a list of partitions after.