[8.x] Ensure $prefix is a string

[kylekatarnls]

Because PHP 8.1 will no longer allow it and throw:
rtrim(): Passing null to parameter #1 ($string) of type string is deprecated

[kylekatarnls]

To reproduce the error just run the following with last master PHP build:

new \Illuminate\Routing\Route('GET', '/foo', []);

It actually calls prefix(null) which makes me think the PHPDoc likely should have @param string|null $prefix if it's supposed to be a normal case.

[driesvints]

@kylekatarnls I think we should fix the prefix call instead as this method shouldn't ever be passes a value of null