Flutter: How to create json from model that only contains non null or not empty values

I need to create a json from model. But if some field was empty or null in my model my json should not contain that specific key at all.

I need to create a json object like this:

{
    "bloodTest": ["640dd3ed11aeebf5200dd319"],
    "weight": 100,
    "height": 190,
    "profile": "640dd41a11aeebf5200dd32b"
}

But for example if weight field was empty my json object should be like this:

{
    "bloodTest": ["640dd3ed11aeebf5200dd319"],
    "height": 190,
    "profile": "640dd41a11aeebf5200dd32b"
}

I’ve created my json object by this piece of code in my flutter project:

     var json = {
        "bloodTest": bloodTest ,
        "weight":  info.weight ,
        "height": info.height ,
        "profile": info.profile.id ,
      }

And if some filed is null or empty it still create the json filed with null or empty value.
How should I fix it?