[HttpKernel] do is_file check before include

Trying to include a file that doesn't exist issues a warning. Doing an
is_file check beforehand should prevent those warnings.

src/Symfony/Component/HttpKernel/Kernel.php

@@ -533,7 +533,7 @@ abstract class Kernel implements KernelInterface, RebootableInterface, Terminabl
                 if (!flock($lock, $wouldBlock ? \LOCK_SH : \LOCK_EX)) {
                     fclose($lock);
                     $lock = null;
-                } elseif (!\is_object($this->container = include $cachePath)) {
+                } elseif (!is_file($cachePath) || !\is_object($this->container = include $cachePath)) {
                     $this->container = null;
                 } elseif (!$oldContainer || \get_class($this->container) !== $oldContainer->name) {
                     flock($lock, \LOCK_UN);