minor symfony#28463 KernelInterface can return null for getContainer method (gmponos)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes symfony#28463).

Discussion
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KernelInterface can return null for getContainer method

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

According to [this](https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Client.php#L45) a KernelInterface can return null if it is shutdown.

Commits
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7cb340a KernelInterface can return null container

Author: nicolas-grekas

src/Symfony/Component/HttpKernel/KernelInterface.php

@@ -139,7 +139,7 @@ public function getRootDir();
     /**
      * Gets the current container.
      *
-     * @return ContainerInterface A ContainerInterface instance
+     * @return ContainerInterface|null A ContainerInterface instance or null when the Kernel is shutdown
      */
     public function getContainer();