ktls: Improve handling of the bind_threads tunable a bit

• Only check for empty domains if we actually tried to configure domain affinity in the first place. Otherwise setting bind_threads=1 will always cause the sysctl value to be reported as zero. This is harmless since the threads end up being bound, but it's confusing.
• Try to improve the sysctl description a bit.

Reviewed by: gallatin, jhb
Submitted by: Klara, Inc.
Sponsored by: Ampere Computing
Differential Revision: https://reviews.freebsd.org/D28161