MB-12852: Return correct result from view info

The root node might be empty or the key on the root node might not be
set, hence the MBB needs to be calculated differently.

Change-Id: Ifd4b3752491e602b263a7a5c929dc1534fae3790
Reviewed-on: http://review.couchbase.org/44105
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>