Bug: Event Custom Fields Checkboxes can not be empty

I created event custom fields using checkboxes with 3 options. By default, the checkboxes are not selected. However, after selecting an option and saving the event, it is no longer possible to uncheck all the options and save the event again. It is possible to change the selection but not possible to have empty selection.

Might be a valid issue. I will check and get back to you with the solution on tomorrow (it's midnight here now and I could not check it right away)

Tuan