Ensure mForwardNumber is null if modem returns empty String or null

We populate EXTRA_FORWARDED_NUMBER if mForwardedNumber is non null.
This was happening on some hardware where DriverCall#forwardedNumber
was an empty String instead of null. In either case, we should
set mForwardedNumber to null.

Test: atest FrameworkTelephonyTests:GsmCdmaConnectionTest
Bug: 187826275
Change-Id: Icb225ef012046ef30eb324b666168ce42f2af174