Fix return type of SessionHandler::read()

[kocsismate]

No description provided.

[nikic]

https://www.php.net/manual/en/sessionhandlerinterface.read.php only talks about string return. The implementation seems to treat any non-string return as failure.

I think this change is fine, but how did you run across it?

[kocsismate]

I think this change is fine, but how did you run across it?

When I was working on #6548, I added a lot of return type declarations to child methods of internal classes so that the deprecation notice is avoided. And then I noticed that a session test is failing (I believe it was ext/session/tests/session_set_save_handler_class_005.phpt) because the false return value of the parent::read($id) got converted to "".

[nikic]

I see, thanks.

[kocsismate]

Closing via ffb1ff3