DOC: warning from registry example

The example we show in the documentation issues a W02 warning. Is there anything we can do about it? It's not like the users can, so it should either be captured in the library code itself, or the example reworked so it's not being issued to begin with

cc @msdemlei

Code:

```
from pyvo import registry
resources = registry.search(registry.UCD("src.redshift"), registry.Freetext("supernova"))
resources[4].get_tables()
```

warning:
```
/Users/bsipocz/munka/devel/pyvo/pyvo/io/vosi/vodataservice.py:836: W02: ?:?:?: W02: '' is not a valid datatype according to the VOSI spec
vo_warn(W02, value, self._config, self._pos)
Out[4]: {'J/A+A/437/789/table2': ... 19 columns ...}
```

Reproduce the warning with the documented registry example, then inspect pyvo/io/vosi/vodataservice.py around line 836 and the get_tables() path. Determine whether the library should handle the empty datatype or the example should avoid it; done means the documentation example no longer emits W02.

Written by the indexing model from the issue text.