Add and apply EmptyLineAfterGuardClause rule

We were inconsistent on this one. I consider it particularly useful when
a method starts with a `return` statement.

In other cases, we probably shouldn't have a guard rule in the middle of
a method in any case, but that's a different refactoring.

app/controllers/users/sessions_controller.rb

@@ -15,6 +15,7 @@ class Users::SessionsController < Devise::SessionsController
 
     def verifying_via_email?
       return false if resource.blank?
+
       stored_path = session[stored_location_key_for(resource)] || ""
       stored_path[0..5] == "/email"
     end