Fix potential crash when displaying MMI error result.

Newly added code made the incorrect assumption that an MMI result would
come back as an empty string if not supplied.
We've seen crashes in the field due to a null message string coming in.

Test: Code inspection / compilation.
Test: Wrote unit tests to check null handling in these cases.
Fixes: 191617795
Change-Id: I57d0227fd725f3ae19a2ce7290afc670bd88fd93