Initialize $prefix to suppress Notice Error

$prefix causes Undefined Variable notice error

src/Template/Error/missing_action.ctp

@@ -21,6 +21,7 @@ if (!empty($plugin)) {
     $namespace = str_replace('/', '\\', $plugin);
 }
 $prefixNs = '';
+$prefix = isset($prefix) ? $prefix : '';
 if (!empty($prefix)) {
     $prefix = array_map('\Cake\Utility\Inflector::camelize', explode('/', $prefix));
     $prefixNs = '\\' . implode('\\', $prefix);