MPEG2TSWriter: Read more immediately if an empty buffer is received

If writing encoded data from an OMXCodec, the last buffer can be
an empty buffer (with the EOS flag set, which isn't propagated to
MediaBuffer). If we don't retry reading, we won't ever get the
EOS error and thus won't properly signal that the source is done.

Change-Id: Ibe01dbcd0637fbf0c2529a277e9f208c48e0e8ab