Fix VibratorManager cmd for on-off waveforms

The VibrationEffect.WaveformBuilder.addSustain does not accept zero
duration sustain periods, so adding an extra check to the
VibratorManager ShellCommand implementation to handle on-off patterns
with zero entries.

Bug: 224930189
Test: manual
Change-Id: Ie1b8f15a0e7b029ebf78a2affce1c12455f08418