[5.0] Fix CLI Dependencies

Automatically instantiate a Request object if it's required. (Useful when running from the CLI where it would not already be instantiated.)

ServiceProviders using objects (eg. URLGenerator) which depend on a Request object were squaking due to receiving NULL instead of a Request object.

[See here.](https://laracasts.com/discuss/channels/general-discussion/laravel-5-service-provider)
`if ($request === null)` is faster than a function call.