[{"colors":{"k":null}}]

db.collection.aggregate([{"$set":{"k-cond":{"$cond":[{"$or":[{"$eq":["$colors.k",null]},{"$eq":[{"$type":"$colors.k"},"missing"]}]},"","$colors.k"]},"k-ifnull":{"$ifNull":["$colors.k",""]},"k-if":{"$cond":["$colors.k","$colors.k",""]}}}])