dnsdist: Properly handle ECS for queries with ancount or nscount > 0

[rgacogne]

Short description

Some queries (some NOTIFY, for example) have non-empty answer section. Our code handling the addition of EDNS Client Subnet was not properly handling that case when there was no existing EDNS in the query, or more precisely when the additional section was empty.
Closes #10409.
Supersedes #10410.

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)