[5.2] Add support ID with 0 for value CheckboxField.php (joomla#37174)

Author: korenevskiy

libraries/src/Form/Field/CheckboxField.php

@@ -137,7 +137,8 @@ public function setup(\SimpleXMLElement $element, $value, $group = null)
     protected function getLayoutData()
     {
         $data            = parent::getLayoutData();
-        $data['value']   = $this->default ?: '1';
+        // Allow any non-empty string, such as '0', to be used as the default value for a checkbox
+        $data['value']   = $this->default !== null && $this->default !== '' ? $this->default : '1';
         $data['checked'] = $this->checked || $this->value;
 
         return $data;