fix harelba/q#62

Hello, thanks for your time and efforts maintaining q

I was hurted by #62 so here is my patch

Sorry, it breaks test_one_row_of_data_with_header_param

Also does not produce 'Warning - data is empty', but it is intentional:
- there is no way to hide warnings whithout filtering real errors (all in stderr)
- empty data is nominal case to me, especially for LEFT JOIN

So let's discuss what can be done

Thanks