lib/flua/libjail: Allow empty params table

The name or jid always gets added to the params, and that's enough to
avoid allocating a 0 length params array.

Reported by: kevans
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D28778

(cherry picked from commit e175b519a6fb83889fb3ca679b73d11ea5bea7ad)