[13.x] Fix nullable return types on the route binding registrar

[dfinchenko]

Illuminate\Contracts\Routing\BindingRegistrar::getBindingCallback() documents @return \Closure, but its only implementation returns null whenever the given key has no registered binder:

public function getBindingCallback($key)
{
    if (isset($this->binders[$key = str_replace('-', '_', $key)])) {
        return $this->binders[$key];
    }
}

Broadcaster::binder() has the same gap. It documents @return \Illuminate\Contracts\Routing\BindingRegistrar, but assigns null when no registrar is bound in the container:

$this->bindingRegistrar = Container::getInstance()->bound(BindingRegistrar::class)
    ? Container::getInstance()->make(BindingRegistrar::class)
    : null;

Because both docblocks claim non-null, static analysis reports the framework's own null handling as dead code. Running PHPStan over the contract, the router and the broadcaster (level 6, checkPhpDocMethodSignatures: true):

Broadcaster.php:231: Left side of && is always true.
Broadcaster.php:231: Right side of && is always true.
Broadcaster.php:235: Unreachable statement - code above always terminates.
Broadcaster.php:300: Negated boolean expression is always false.

Those are the if ($binder && $binder->getBindingCallback($key)) guard and the return $value; fallback in resolveExplicitBindingIfPossible(), plus the lazy initialization in binder(). Adding |null to the three docblocks clears all four.

The Route facade already documents the real type: @method static \Closure|null getBindingCallback(string $key).

Docblocks only, no behavior change.