Add a null check for mDefaultQos in DataCallResponse.writeToParcel()

java.lang.NullPointerException:
Attempt to invoke virtual method 'int android.telephony.data.Qos.getType()'
on a null object reference

Bug: 173667436
Test: Basic data and Iwlan/VoLTE handover test.
Change-Id: I6045ac9fa4e757eea74389002d0d443d20459d46