RESTHandler getopt_usage: schema properties can be undef

RESTHandler getopt_usage: schema properties can be undef

While seldom some of our API endpoints do not define it, e.g., the
world readable /access/ticket call.

As all of the stack can cope with that just fine make getopt_usage
also follow that behavior and don't assume that properties has to be
defined.

This fixes a complaint about undefined value use in the following
calls:
 pvesh usage /access/ticket
 pmgsh help /access

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>