BaseHtml::activeListBox() incorrect 'unselect' value fixed.

framework/helpers/BaseHtml.php

@@ -1276,7 +1276,7 @@ class BaseHtml
 		$name = isset($options['name']) ? $options['name'] : static::getInputName($model, $attribute);
 		$selection = static::getAttributeValue($model, $attribute);
 		if (!array_key_exists('unselect', $options)) {
-			$options['unselect'] = '0';
+			$options['unselect'] = '';
 		}
 		if (!array_key_exists('id', $options)) {
 			$options['id'] = static::getInputId($model, $attribute);