Use `next` instead of `break`; avoid terminating whole loop

We want to avoid terminating the whole loop here, because it will cause
parameters that should be removed to not be removed, since we are
terminating early. In this specific case, `param2` is processed before
`param1` due to the reversing of `route.parts`, and since `param2` fails
the check on this line, it would previously cause the whole loop to
fail, and `param1` would still be in `parameterized_parts`. Now, we are
simply calling `next`, which is the intended behavior.

Introduced by 8ca8a2d7.

Fixes #27454.

actionpack/lib/action_dispatch/journey/formatter.rb

@@ -36,7 +36,7 @@ def generate(name, options, path_parameters, parameterize = nil)
 
           route.parts.reverse_each do |key|
             break if defaults[key].nil? && parameterized_parts[key].present?
-            break if parameterized_parts[key].to_s != defaults[key].to_s
+            next if parameterized_parts[key].to_s != defaults[key].to_s
             break if required_parts.include?(key)
 
             parameterized_parts.delete(key)

actionpack/test/controller/url_for_test.rb

@@ -487,6 +487,27 @@ def test_url_for_with_array_is_unmodified
         end
       end
 
+      def test_default_params_first_empty
+        with_routing do |set|
+          set.draw do
+            get "(:param1)/test(/:param2)" => "index#index",
+              defaults: {
+                param1: 1,
+                param2: 2
+              },
+              constraints: {
+                param1: /\d*/,
+                param2: /\d+/
+              }
+          end
+
+          kls = Class.new { include set.url_helpers }
+          kls.default_url_options[:host] = "www.basecamphq.com"
+
+          assert_equal "http://www.basecamphq.com/test", kls.new.url_for(controller: "index", param1: "1")
+        end
+      end
+
       private
         def extract_params(url)
           url.split("?", 2).last.split("&").sort