RedirectIfAuthenticated: Allow redirectUsing to accept null values for resetting behavior

[Sagmedjo]

Description:

This change introduces an update to the RedirectIfAuthenticated middleware. The goal is to allow resetting the middleware's behavior to its default state by passing null to the redirectUsing method.

Currently, once a custom callback is set using redirectUsing(callable $redirectToCallback), there is no direct way to reset it to the default behavior. This enhancement allows null values to be passed, effectively resetting the middleware to its original redirection behavior.

The redirectUsing method has been updated as follows:

// Original method signature
public static function redirectUsing(callable $redirectToCallback)
{
    static::$redirectToCallback = $redirectToCallback;
}

// Updated method signature to allow null
public static function redirectUsing(?callable $redirectToCallback)
{
    static::$redirectToCallback = $redirectToCallback;
}

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!