INFO value attribute must be present, but it could be empty

My (datalink) votable contains an INFO element with the value attribute present, but left empty, as the value is placed in the INFO body, as shown here:

``

The issue is that when I build a datalink object from that votable:

`datalink = vo.dal.adhoc.DatalinkResults.from_result_url(url)`

I get a warning:

```
WARNING: W35: None:6:6: W35: 'value' attribute required for INFO elements [astropy.io.votable.tree]
WARNING:astropy:W35: None:6:6: W35: 'value' attribute required for INFO elements
```

That warning is incorrect, as it is true that the INFO element in a votable requires the value attribute to be present, but nowhere in the VOTable specs is written that the value attribute cannot be an empty string.

As soon as I change from `value=""` to `value=" "` the warning disappears. Of course I could implement a space instead of an empty string, but I think it would be better to have pyvo fixed.

Thanks!