don't send zero-sized binaries to xdcr upr streamer consumer end

Because it's useless. And it's not that rare, particularly when full
upr packet is longer than tcp segment.

Change-Id: I2b44a63b488d04da3e74c8d4f81b2f4205d07e76
Reviewed-on: http://review.couchbase.org/35428
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>