[10.x] Fix Dispatcher::until return type

[Neol3108]

It appears the until method always returns whatever gets returned from a listener or null if nothing is returned from any listener.

As you can see the until method returns the result of the dispatch method with parameter $halt = true:
https://github.com/laravel/framework/blob/10.x/src/Illuminate/Events/Dispatcher.php#L222

And the behaviour of dispatch with $halt = true is as follows:

• If a listener returns something other than null, that value will be early returned. https://github.com/laravel/framework/blob/10.x/src/Illuminate/Events/Dispatcher.php#L254
• If no listener returned anything, it will return null https://github.com/laravel/framework/blob/10.x/src/Illuminate/Events/Dispatcher.php#L268

I was considering adding a conditional return type to the dispatch method based on the halt boolean. I however thought they weren't used anywhere else in the framework and decided to omit this. Please let me know if this is wanted.