Invalid XML Data set does not throw an appropriate exception

If you define a XML Result set during a JDBC response, it should look like this:
```xml

value

```

If you leave out the `row` tag, the `XmlDataSetProducer` does not throw an appropriate exception, which states that the dataset specification is invalid.

Instead, a dataset with empty rows is produced causing a null pointer during data access in the result set.

This makes it hard for the user to find out, that the dataset specification is invalid.

Start with XmlDataSetProducer, which handles the XML dataset used during a JDBC response, and trace how input without a row element is parsed. Reproduce the missing-row case and check the result-set access path; done means invalid dataset XML raises an appropriate exception instead of producing empty rows that later cause a null pointer.

Written by the indexing model from the issue text.