CCBC-405: Return error code if passed an empty key

Empty keys will result in the server giving us an EINVAL and confuse the
user. We should notify the user about this early on by supplying an
appropriate error code.

Change-Id: Ifd59293d8f5eea0304017859bff3aa2e80a70cb8
Reviewed-on: http://review.couchbase.org/39284
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>