Avoid calling to_s on nil in journey/formatter

When `defaults[key]` in `generate` in the journey formatter is called, it often returns a `nil` when we call `to_s` on a nil, it allocates an empty string. We can skip this check when the default value is nil.

This change buys us 35,431 bytes of memory and 887 fewer objects per request.

Thanks to @matthewd for help with the readability

actionpack/lib/action_dispatch/journey/formatter.rb

@@ -32,8 +32,8 @@ def generate(name, options, path_parameters, parameterize = nil)
 
           defaults       = route.defaults
           required_parts = route.required_parts
-          parameterized_parts.delete_if do |key, value|
-            value.to_s == defaults[key].to_s && !required_parts.include?(key)
+          parameterized_parts.keep_if do |key, value|
+            defaults[key].nil? || value.to_s != defaults[key].to_s || required_parts.include?(key)
           end
 
           return [route.format(parameterized_parts), params]