fix #default_app on rack 3.2.0

Rack::Builder#run returns nil, see rack/rack#2355

Author: theodorton

test/propshaft/server_test.rb

@@ -87,8 +87,9 @@ def call(env)
 
   private
     def default_app
-      builder = Rack::Builder.new
-      builder.run @server
+      Rack::Builder.new.tap do |b|
+        b.run @server
+      end
     end
 
     def app