Changes

trxcon/l1sched: make l1sched_lchan_emit_data_cnf() NULL-safe

Passing NULL to l1sched_lchan_emit_data_cnf() is not normal and
generally not expected, but definitely not fatal enough to abort
the process completely (due to assertion failure).

Change-Id: Ie64c176265f66a6c1515c66eb465d7e60f6768db