…` (elementaire)

This PR was merged into the 6.3 branch.

Discussion
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[HttpKernel] Handle nullable callback of `StreamedResponse`

| Q             | A
| ------------- | ---
| Branch?       | 6.3
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       |  -
| License       | MIT

This PR fixes a bug introduced by #51396. Having a callback in `StreamResponsed` is not a mandatory by design.

So the method `getCallback` must check it like `sendContent` does.

When we are dealing with cacheable ressource, only headers are sent from `StreamResponsed`, for example:

```php
#[Route(path: '/download')]
public function download(Request $request): StreamedResponse
{
    // Get the last modified of a file.
    $lastModified = \DateTime::createFromFormat('d/m/Y H:i:s', '10/10/2023 13:22:00');

    $response = (new StreamedResponse())->setLastModified($lastModified);

    if ($response->isNotModified($request)) {
        return $response;
    }

    // Get the stream of a file and attach it to the callback.
    // ...

    return $response;
}
```

`HttpKernel` class have to handle it else if you obtain `HTTP 500` with message `Value of type null is not callable`.

cc `@nicolas`-grekas

Commits
-------

66648c5 [HttpKernel] Handle nullable callback of StreamedResponse