Unchecked value should always be a string if not null

This makes it consistent with hidden input values in HTML. Which is how unchecked values are used.

Relevant snippet In check_box.rb,

module FormProps
  module Inputs
    class CheckBox < Base
      def initialize(object_name, method_name, template_object, checked_value, unchecked_value, options)
        @checked_value = checked_value
        @unchecked_value = unchecked_value
        super(object_name, method_name, template_object, options)
      end

Start in check_box.rb at FormProps::Inputs::CheckBox#initialize, where @checked_value and @unchecked_value are assigned. Trace how the unchecked value is rendered and verify the behavior against the issue's HTML hidden-input expectation. Done means every non-null unchecked value is represented as a string while null remains null.