policychecker: allow signal rules if send_path is present

policychecker: allow signal rules if send_path is present

In case a rule contained send_path, we required it also to have
send_destination or send_destination_prefix.
However, a rule may be crafted for signals, which are broadcast,
therefore they don't have any destination at all.
This patch relaxes the requirement a bit, allowing rules
having send_path without send_destination under condition that
the rule contains send_interface and send_type='signal' attributes.

Change-Id: I99ff19271121f726df81e2aed8a124a78f7eadbc