…ty (yceruto)

This PR was merged into the 4.2 branch.

Discussion
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[HttpKernel] Fix get session when the request stack is empty

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

This bug happen behind an exception on a kernel response event, when one collector (e.g. `RequestDataCollector`) is trying to get the request session and the request stack is currently empty.

**Reproducer**
https://github.com/yceruto/get-session-bug (`GET /`)

See logs on terminal:
```bash
Apr 15 20:29:03 |ERROR| PHP    2019-04-15T20:29:03-04:00 Call to a member function isSecure() on null
Apr 15 20:29:03 |ERROR| PHP    PHP Fatal error:  Uncaught Symfony\Component\Debug\Exception\FatalThrowableError: Call to a member function isSecure() on null in /home/yceruto/demos/getsession/vendor/symfony/http-kernel/EventListener/SessionListener.php:43
Apr 15 20:29:03 |DEBUG| PHP    Stack trace:
Apr 15 20:29:03 |DEBUG| PHP    #0 /home/yceruto/demos/getsession/vendor/symfony/http-kernel/EventListener/AbstractSessionListener.php(59): Symfony\Component\HttpKernel\EventListener\SessionListener->getSession()
Apr 15 20:29:03 |DEBUG| PHP    #1 /home/yceruto/demos/getsession/vendor/symfony/http-foundation/Request.php(707): Symfony\Component\HttpKernel\EventListener\AbstractSessionListener->Symfony\Component\HttpKernel\EventListener\{closure}()
Apr 15 20:29:03 |DEBUG| PHP    #2 /home/yceruto/demos/getsession/vendor/symfony/http-kernel/DataCollector/RequestDataCollector.php(65): Symfony\Component\HttpFoundation\Request->getSession()
Apr 15 20:29:03 |DEBUG| PHP    #3 /home/yceruto/demos/getsession/vendor/symfony/http-kernel/Profiler/Profiler.php(167): Symfony\Component\HttpKernel\DataCollector\RequestDataCollector->collect(Object(Symfony\Component\HttpFoundation\Request), Object(Symfony\Component\HttpFoundation\Respo in /home/yceruto/demos/getsession/vendor/symfony/http-kernel/EventListener/SessionListener.php on line 43
```

Friendly ping @nicolas-grekas as author of the previous PR symfony/symfony#28244

Commits
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d62ca37 Fix get session when the request stack is empty