Avoid duplication in r.bridgetown implementation (#939)

By choosing an appropriate argument to `on`, based on the
base path, we don't need a separate if statement block and
manual return.

Additionally, the nil at the end is not needed, as `on` will either throw if it handles the request, or return nil if it does not.

bridgetown-core/lib/roda/plugins/bridgetown_server.rb

@@ -158,20 +158,11 @@ def bridgetown
           scope.initialize_bridgetown_context
           scope.initialize_bridgetown_root
 
-          # There are two different code paths depending on if there's a site `base_path` configured
-          if Bridgetown::Current.preloaded_configuration.base_path == "/"
+          base_path = Bridgetown::Current.preloaded_configuration.base_path.delete_prefix("/")
+          on(base_path.empty? ? true : base_path) do
             ssg # static file server
             Bridgetown::Rack::Routes.load_all scope
-            return
           end
-
-          # Support custom base_path configurations
-          on(Bridgetown::Current.preloaded_configuration.base_path.delete_prefix("/")) do
-            ssg # static file server
-            Bridgetown::Rack::Routes.load_all scope
-          end
-
-          nil
         end
       end
     end