How does one conditionally get null as a default value in cookiecutter prompts?

Here is my example cookiecutter.json and Jinja logic

{
    "pick_one": [
        "Choice A",
        "Choice B"
    ],
    "user_name": "{%- if cookiecutter.pick_one == 'Choice A' -%} mydefault {%- else -%}  null {%- endif -%}",
    "full_name": null
}

Notice that full_name's default value is null, and this means an entry from the user is required.

For user_name, I'd like to have one default based on the pick_one value or null if not that value. In other words, If Choice A is selected, I'd like the default to be mydefault and if not, I'd like null to be the default and user_name to work the same as full_name does in the example.

So far, I've tried wrapping null in curly braces, added safe to it. {{ null }} or {{ null | safe }} but I get an error that the variable cannot be rendered because 'null' is undefined. In other attempts, I get the string "null" as a default instead of requiring the user to enter a value.

Thanks so much!