Fix improper usage of scheduler exposed by 5c713fdf

When 5c713fdf was merged, it allowed for scheduled items to have an ID of
'0' returned. While this was valid per the documentation for the API, it was
apparently never returned previously. As a result, several users of the
scheduler API viewed the result as being invalid, causing them to reschedule
already scheduled items or otherwise fail in interesting ways.

This patch corrects the users such that they view '0' as valid, and a returned
ID of -1 as being invalid.

Note that the failing HEP RTCP tests now pass with this patch. These tests
failed due to a duplicate scheduling of the RTCP transmissions.

ASTERISK-25449 #close

Change-Id: I019a9aa8b6997584f66876331675981ac9e07e39