arcstat: prevent ZeroDivisionError when L2ARC becomes empty

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard@ryao.dev>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #17348