Only use path if the key is not present

There are some cases where :path is nil on option and we should respect
that.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -881,7 +881,7 @@ def namespace(path, options = {})
             shallow_prefix: options.fetch(:as, path)
           }
 
-          path_scope(options.delete(:path) || path) do
+          path_scope(options.delete(:path) { path }) do
             scope(defaults.merge!(options)) { yield }
           end
         end