PYCBC-131: More informative error on empty string

Currently when an empty string is passed, lcb throws an InvalidError,
which works but isn't expected for the SDK - we should explicitly detect
an empty string and warn about this. It's not difficult to do!

Change-Id: Ief383d8557531313995569e3f4368cfdb347969c
Reviewed-on: http://review.couchbase.org/26991
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>