AudioSessionOut: Fix to handle a single write() with an empty buffer

- This fix is a workaround to exit cleanly in the case where the AudioSink
  write might give just a single zero length buffer to the driver.

Change-Id: I68d023e6f0bd5b5e4f593a42b26c685e3405288b
CRs-Fixed: 445182
Merge "AudioSessionOut: Fix to handle a single write() with an empty buffer" into cm-10.1