Fix PUBDEV-2933: fold_assignment="Stratified" now works with NA in the response.
Fix PUBDEV-2933: fold_assignment="Stratified" now works with NA in the response.

michalkurka/h2o-3,jangorecki/h2o-3,h2oai/h2o-3,h2oai/h2o-dev,mathemage/h2o-3,h2oai/h2o-3,mathemage/h2o-3,michalkurka/h2o-3,jangorecki/h2o-3,mathemage/h2o-3,mathemage/h2o-3,spennihana/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,mathemage/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,jangorecki/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,spennihana/h2o-3,spennihana/h2o-3,h2oai/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,mathemage/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,jangorecki/h2o-3,jangorecki/h2o-3,mathemage/h2o-3,michalkurka/h2o-3,jangorecki/h2o-3,spennihana/h2o-3,h2oai/h2o-3,jangorecki/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,h2oai/h2o-3