cast variable

administrator/components/com_fields/models/field.php

@@ -217,7 +217,7 @@ public function save($data)
 	private function checkDefaultValue($data)
 	{
 		// Empty default values are correct
-		if (empty($data['default_value']))
+		if (empty($data['default_value']) && $data['default_value'] !== "0")
 		{
 			return true;
 		}

libraries/src/Form/Rule/OptionsRule.php

@@ -42,7 +42,7 @@ public function test(\SimpleXMLElement $element, $value, $group = null, Registry
 		// Check if the field is required.
 		$required = ((string) $element['required'] == 'true' || (string) $element['required'] == 'required');
 
-		if (!$required && empty($value))
+		if (!$required && empty($value) && !($data['default_value'] !== "0"))
 		{
 			return true;
 		}