Change error to no error, when speak data is empty

Change error to no error, when speak data is empty

Cause: In the __wait_synthesis() function, if the synth_control is changed from DOING
to DONE and there's no next speak_data, the synthesis is terminated. At this time, it
is normal operation, even if there's no speak data.

Solution: Change error to no error, when speak data is empty.

Reference log:
01-27 05:01:43.733+0900 I/ttsd     (P  798, T  798): ttsd_server.c: __wait_synthesis(100) > [Server] synth_control(1)
01-27 05:01:43.733+0900 D/ttsd     (P  798, T  798): ttsd_server.c: __synthesis(116) > @@@ SYNTHESIS  START
01-27 05:01:43.733+0900 W/ttsd     (P  798, T  798): ttsd_data.cpp: ttsd_data_get_speak_data(563) > [DATA WARNING] There is no speak data
01-27 05:01:43.733+0900 D/ttsd     (P  798, T  798): ttsd_server.c: __synthesis(121) > @@@ Fail to get speak data. ret(-49348604)
01-27 05:01:43.733+0900 D/ttsd     (P  798, T  798): ttsd_server.c: __synthesis(122) > @@@ SYNTHESIS  END

Change-Id: I8ce4823ee9791b1b2854c1f1ed7c01469151807e