[{"_id":"123dsadasfa454sdsaw","hashmap":{"uuid-12sadsadw5":{"name":"bob"},"uuid-12sadsadwew5":{"name":"alice"},"uuid-other":{}},"age":10}]

db.collection.aggregate([{"$set":{"hashmap":{"$objectToArray":"$hashmap"}}},{"$unwind":"$hashmap"},{"$match":{"hashmap.v.name":{"$ne":null}}},{"$group":{"_id":"$_id","hashmap":{"$push":"$hashmap"}}},{"$set":{"hashmap":{"$arrayToObject":"$hashmap"}}}])