Avoid wrapping Artisan command --url option with url() helper when possible

[bakerkretzmar]

This PR closes #359, but note that it doesn't fix that particular issue—the cause of that issue is an invalid APP_URL, which is a case that Laravel and Ziggy do not officially support.

What that issue raised, though, was the fact that we're now applying the url() helper inconsistently. In #334 we updated the Ziggy class to prefer APP_URL over the url() helper, but I missed the fact that the CommandRouteGenerator class was still always wrapping its --url option with the url() helper—even when the --url option is null.

This PR ensures that if no --url option is passed, the generate command passes that null option on to the Ziggy class to handle properly.

Before

• Not passing a --url option to php artisan ziggy:generate uses url('/')
• Passing a valid URL to the --url option uses that URL directly
• Passing anything else to the --url option uses url($this->option('url'))

After

• Not passing a --url option to php artisan ziggy:generate uses config('app.url', url('/'))
• Passing a valid URL to the --url option uses that URL directly (unchanged)
• Passing anything else to the --url option uses url($this->option('url')) (unchanged)