ListField does not respect value None (default value is []).

Basically, there is no way to get rid of a ListField element in output json.
For other fields, you set them to None and it will be removed from output of to_struct(), but this does not apply to ListField. I reckon, ideally, None value should remove it from json and [] should put a [] in json.
It looks like the default value for ListField is [] so there is no way for to_struct() to understand whether user set the field to None, [] or left it unassigned.
Also, it looks like 'required' and 'nullable' properties are ignored for ListField.