dns/eve: use default formats if formats is empty

dns/eve: use default formats if formats is empty

If the configuration field "formats" is empty, DNS response records do
not have any relevant information other than that there was a
response, but not much about the response.

I'm pretty sure the intention here was to log the response details if
no formats were provided, which is what happens when the field is
commented out.

So if no formats are specified, use the default of all.

Bug: #6420