Fix NPE in TTS service if no engine string

Test: added test in TextToSpeechServiceTest
Bug: 232438498
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e5364f93d3208a66d599d7256d2e4cfaba989e0e)
Merged-In: Ia587d34e2898930c0956024c56a1f19debb445e4
Change-Id: Ia587d34e2898930c0956024c56a1f19debb445e4
1 file changed