libosmo-trx/trxd: support NOPE.{ind,req} in TRXDv0/v1 PDUs
TRXDv0 (and TRXDv1 in the downlink direction) has no MTS field, but NOPE indications/requests do exist there in practice: the burst payload is simply omitted, i.e. a header-only PDU. This is how osmocom-bb's trxcon transmits NOPE.req (see trx_if_handle_phyif_burst_req()).
