v4l2_m2m devices may send an empty packet/frame while draining
to indicate that all capture buffers have been flushed.

Currently, the empty packet/frame is not handled correctly:
When encoding, the empty packet is forwarded to the muxer, usually
creating warnings.
When decoding, a reference to the memory is created anyway. Since in
the past this memory contained a decoded frame, it results in an extra
frame being decoded.

This commit discards the empty packet/frame.

References:
linux/Documentation/media/uapi/v4l/dev-decoder.rst:

    "The last buffer may be empty (with :c:type:`v4l2_buffer` bytesused = 0)
     and in that case it must be ignored by the client, as it does not
     contain a decoded frame."

linux/Documentation/media/uapi/media/v4l/vidioc-encoder-cmd.rst:

    "...This buffer may be empty, indicated by the
     driver setting the ``bytesused`` field to 0."
Reviewed-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>