Adding a check for '' and failure. When trying to Cache '', FileEngin…

…e will return false. This shouldn't trigger an error inside Cache.

cake/libs/cache.php

@@ -270,7 +270,7 @@ public static function write($key, $value, $config = null) {
 
 		$success = self::$_engines[$config]->write($settings['prefix'] . $key, $value, $settings['duration']);
 		self::set();
-		if ($success === false) {
+		if ($success === false && $value !== '') {
 			trigger_error(
 				sprintf(__("%s cache was unable to write '%s' to cache", true), $config, $key),
 				E_USER_WARNING