Ensure empty string MMI responses default to "MMI Complete".

The existing code checks for null only, but it looks like the modem or
network can sometimes return empty string when there is not result,
which would cause no "MMI Complete" dialog to show.
Checking for empty string as well will solve this problem.

Test: Regression tested MMI codes.
Bug: 37561464
Change-Id: I09caec7eae944bd3dfe97ac8f89c8a113800de1e