Revert "Update routing test handles params without a value"

Rack v3.0.7 rolled back this change with
rack/rack#2059

Other references:

- rails/rails#47652
- rack/rack#2052
- rack/rack#2038

test/routing_test.rb

@@ -313,7 +313,7 @@ class RoutingTest < Minitest::Test
   it "handles params without a value" do
     mock_app {
       get '/' do
-        assert_empty params.fetch('foo')
+        assert_nil params.fetch('foo')
         "Given: #{params.keys.sort.join(',')}"
       end
     }