[HttpKernel] Handle nullable callback of StreamedResponse

[elementaire]

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:

#[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

EDIT: found by @alexismarquis

[nicolas-grekas]

Thank you @elementaire.

[stof]

I would use null === $this->callback for consistency with our other null checks

[nicolas-grekas]

isset() is what we use on 7.0 because the property is not declared as nullable

[stof]

but this whole ticket is about fixing the fact that the callback can be null. So to me, the 7.0 type should also be changed as part of this fix.

[stof]

btw, as the property is protected and not private, this is a BC break to turn it into a non-nullable potentially-uninitialized property as the way to represent an existing state.