Make catalog 'extra' optional

For source catalog jsons, this leads to an error ('extra argument missing')
```
[
  {
    "ra": 90.222,
    "dec": -12.34,
    "name": "Favourite",
]
```

but this works:

```
[
  {
    "ra": 90.222,
    "dec": -12.34,
    "name": "Favourite",
    "extra": {    }
]
```

For large catalogs, it might be extraneous to add that empty dictionary, so could consider skipping if not found.