trx: scheduler: Keep RTP clock up to date while in DTXu pause
trx: scheduler: Keep RTP clock up to date while in DTXu pause

Upper layer requires us to trigger all events coming from lower layers
in order to keep the RTP clock in sync. In this case, this is done by
sending an empty payload to indicate there's no data to send.

Change-Id: I0bdfb529f35253ca7e531bb9984a3839c3bfe7e8

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