7.2.2.4.2 Conditions for Sending SEID=0 in PFCP Header
Even if the session hasn't been found when processing the N4 modification request, UPF should send the N4 modification
response with cause PFCP_CAUSE_SESSION_CONTEXT_NOT_FOUND and SEID = 0 to SMF still.
UPF PR #47
According to TS 29.244:
Even if the session hasn't been found when processing the N4 modification request, UPF should send the N4 modification response with cause
PFCP_CAUSE_SESSION_CONTEXT_NOT_FOUND
and SEID = 0 to SMF still.