Checkbox value should be 0 if not checked

[jamminjames]

Describe the bug
The value of a checkbox is 1 if selected, but not recorded at all if not checked, but it should be 0. This is pretty standard for checkboxes, and necessary.

This way, if the checkboxes are included in the user's profile, for example, and they uncheck something because they don't want it anymore (for example, they choose not to be on a mailing list anymore), the value will change in the database. Right now, it does not change.

WordPress Environment

``` Latest WordPress and Paid Memberships Pro versions, PHP 7.4 ```