[3.x] Wrap login link in app.stub with a route check

[Quezler]

personally i often run this with the --auth flag just to get this stub and delete some of the files i don't need for that project.

this small change would prevent the layout file from throwing an exception when you've removed the Auth::routes from the web routing file.

(+ the register one below it already has the check too and is enabled by default anyways, just like login is)

feel free to either merge this or close it, i'll be happy either way but i thought i would just leave this here to be nice 😃