The lines from 2323 to 2337 shows to me that internal the 0 is used for disabling the feature if no avp is set. https://github.com/kamailio/kamailio/blob/master/src/modules/rtpproxy/rtppro...
So the check could be changed to: `ice_candidate_priority_val.n < 0 || ice_candidate_priority_val.n > 2`