How to store the absence of a value in Monge if populations are used?

Question

MongoDB

In the Mongoose model, one of the values ​​looks like this

image: { type: mongoose.Schema.Types.ObjectId, ref: 'Image' }

Those. ObjectId is stored in the database, which points to an object from another collection.
What if this field is not required?
If I store null, then I'm at a dead end on the front how to handle a bunch of conflicts (the data is used in SPA in a dozen components and knocks out an error on the client Cannot read property 'slug' of null
like If I store false, then nothing flies from the API at all, an error like

Cast to ObjectId failed for value \"false\" at path \"_id\" for model \"Image\"

It would be a good option with undefined, but I can't send it undefinedfrom the client - an object with this value is simply ignored and is not sent even by Ajax to the server.
What to do?