Ensure call forwarding and barring MMI codes can work over IMS.

The code included a mDialingNumber != null check which would prevent
call forwarding and call barring codes from ever being handled over IMS.
E.g. *30#6505551212 should be handled because it is a valid SC_CLIP
request, but the 6505551212 dialing number would prevent this from going
over IMS.

Test: Dialed MMI code and verified it goes over IMS.
Change-Id: I4bc93085a9c012d80fcce67c65b851995cfe8996
Fixes: 38439766