The --no-run-if-empty option is a GNU extension and the long version isn't supported by *BSD variants.
Instead use the short version (-r) which is present, but ignored as it is the default behavior, in at least FreeBSD: https://www.freebsd.org/cgi/man.cgi?xargs
Most specific zone selected by deepest sub-domain (how many '.' in the domain)
rather than seemingly irrelevant count of the number of characters within the zone.
neil