dnsdist: Prevent EOF error for empty console response w/o sodium

dnsdist: Prevent EOF error for empty console response w/o sodium

Prevent the "Fatal error: EOF while reading message" error when
the server send an empty response to a single console command.
It was caused by calling `readn2()` for a 0-sized read.
When libsodium support is enabled, padding prevents 0-sized
responses anyway.