l1sap.c: Avoid sending RTP frame with empty payload
l1sap.c: Avoid sending RTP frame with empty payload

Depends on libosmo-abis Id6099372b6231c0a4b6ea0716f46f5daee7049e1

Change-Id: Ie9053674aa4f43aac20dbd5c865d70317360abbc

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts