Changes

ngap: Add template for UEContextReleaseReq without PDU Session List

That's the only optional IE in the message. Add a separate template to
be able to transmit with that IE set, similar to what's already done in
m_n2_InitialContextSetupRequest_withPDUSessionList().

Change-Id: I0f1a549103b70bcd373dcd4c688761d3eeef10a6