dnsdist: Handle empty DNSNames in grepq()

[rgacogne]

Short description

We should not get an empty DNSName into the ring buffer, but let's
handle it gracefully if it does happen so we can investigate, instead
of getting:

Caught exception: empty dnsnames aren't part of anything

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)