Fixed a crash in settings in tts engine selection screen.

The crash happens when you try to uninstall current tts
engine and install a different one and then engine name
may be null.

If engine name is null then you can't select new engine
because of that crash, so that whole tts feature becomes
unusable.

Change-Id: Ie31a947eb6444242e8536db2dae1452ef74cc1ce