Fix default Custom Field in some cases (#27618)

Author: HLeithner

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

@@ -26,7 +26,7 @@ class OptionsRule extends FormRule
 	 * Method to test the value.
 	 *
 	 * @param   \SimpleXMLElement  $element  The SimpleXMLElement object representing the `<field>` tag for the form field object.
-	 * @param   mixed              $value    The form field value to validate.
+	 * @param   mixed              $value    The value to validate.
 	 * @param   string             $group    The field name group control value. This acts as an array container for the field.
 	 *                                       For example if the field has name="foo" and the group value is set to "bar" then the
 	 *                                       full field name would end up being "bar[foo]".
@@ -42,7 +42,10 @@ 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))
+		// Check if the value is empty.
+		$blank = empty($value) && $value !== '0' && $value !== 0 && $value !== 0.0;
+
+		if (!$required && $blank)
 		{
 			return true;
 		}