Added ability to remove the prefix from default short URLs

[ash-jc-allen]

This PR adds the ability to create short URLs with an empty prefix.

If the short-url.prefix config field is set to null, a prefix won't be used in the default short URL when it's stored in the database.

I've not made any changes to the default route in web.php because it appears that Laravel strips double forward slashes, so we don't need to handle this ourselves (as far as I can tell).

[wing5wong]

null prefix appears to cause 404 when using non default route.

Route::get('{shortURLKey}', '\AshAllenDesign\ShortURL\Controllers\ShortURLController'); route defined anywhere in routes file causes all routes to return 404

any route with any other prefix will work but not basic routes