Protect against NPE during RTT call sessions

If the ImsCall moves to null during RTT session
we do not guard against against using the now
null reference, which can cause an NPE.

Guard against this condition as is done in the
rest of the class when this condition occurs.

Fixes: 233779563
Test: atest FrameworksTelephonyTests; verify calling on TMO
Change-Id: I92b7af69d2f51d4c32016ccc2a8972d338ba17ce