Fix checkbox value validation, allow for empty checkbox posting.

Author: Liran Cohen

src/Models/CustomField.php

@@ -41,7 +41,7 @@ private function fieldValidationRules()
                 'integer',
             ],
             'checkbox' => [
-                'in:on',
+                'in:0,1',
             ],
             'radio' => [
                 'string',