"false" should be a valid value for a "required" boolean checkbox

[ndbroadbent]

I saw the docs for the Checkbox Field, and I think the code is not properly handling "required" fields:

required | boolean | Indicates whether the field's value is required. If set to true, the field must take on a valid value and cannot be left empty or unassigned.

When a checkbox is unchecked, this should set the field value to "false", and this should be considered a valid value. Right now I am unable to submit the form and the checkbox shows a validation error when it is unchecked.

(Might also be related to #528, but that issue didn't have enough context.)

[ndbroadbent]

Ahhhh, this is the isEmpty function in ListField:

        isEmpty: function()
        {
            return this.data.length === 0;
        },

And Alpaca.Fields.CheckBoxField inherits from Alpaca.Fields.ListField.

Otherwise it would have worked fine if it used the isEmpty function from Field:

        isEmpty: function()
        {
            return Alpaca.isValEmpty(this.getValue());
        },