Optional fields in a Schema? [Arrow JS]

Is there a way I can make some fields in a schema as optional? If I set `nullable` to true then that just adds a default value for the field. Do I instead have the option to not include those fields which do not have any data?
If not, is there a way to set a default value for the fields? For example, a `Bool` field gets a False by default. Can I instead change the default value to `null`?