Fixed empty dns not properly handled issue

1. The original code was not moved correctly when moved
   from data call response into data connection in ag/3296869.
2. Removed the unnecessary stack trace print.
3. Added a new unit test to cover empty dns case.

Test: Unit tests + Telephony sanity tests.
bug: 70506433
Change-Id: Id95304705d3fd0560c0104621c407ad9f8f1148c