Hello. I’m using an extractor from MongoDB and have a stream map for it. However, it encounters errors due to unset values for ‘userId’ in MongoDB. How can I effectively handle these unset values for ‘userId’ and replace it correct value?

config:
      stream_maps:
        spaces: 
          _id : _id
          userId : userId
          createdAt : createdAt
          __else__: __NULL__  
        __else__: __NULL__  
      mongo: