Fix converting method for TrafficDescriptor

In AIDL definition, TrafficDescriptor in DataProfileInfo is
non-null. So it's required to make empty object then return it.
This unmatched nullability could make RPC call drop in binder.
SetDataProfile, SetInitialAttachApn, SetupDataCall is affected.

bug: 205849298
bug: 220644035
Test: manually tested on live network with phone type device which
      enabled HAL 2.0 AIDL

Signed-off-by: Jiwon Kim <jiwon.ss.kim@samsung.com>
Change-Id: I8c03e0d9bf0b8ed103937709ae689203bce6d0d3