MB-11296: don't treat empty value as valid json

Because it's not. It was previously considered valid json due to
"anything and not just object or array is permitted" code that wraps
doc value with square braces. Which obviously caused empty value to be
mis-detected as json.

Change-Id: Id15621cac360047a9353ba0814cb7bbe1c5832b0
Reviewed-on: http://review.couchbase.org/38628
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>