SEND_TO_INCALL null pointer exception fix

A bug was filed on ImsRttTextHandler#handleMessage(Message).
User encountered a null pointer exception in SEND_TO_INCALL case.
2 new conditionals were added to avoid 2 potential null pointer
exceptions and prevent this bug from recurring.

bug: 215215955
Test: 2 Unit tests
Change-Id: I6433b69556b1a913d97db507c17d14a596b4d884
(cherry picked from commit 06dbc439244b659c52edf9400a8dbc8e1b89fa48)
2 files changed