Fix JsonConverterDelegate initialization in RestTestClient

If the RestClient was built with default message converters, then
in mutate, the saved builder also has 0 converters, and adding a
interferes with default registrations.

We need to check if there are no converters at all, and if so
use the default registrations.

See gh-35793

Author: rstoyanchev

spring-test/src/main/java/org/springframework/test/web/servlet/client/DefaultRestTestClient.java

@@ -144,9 +144,10 @@ private static class ConverterCallback {
 			client.mutate()
 					.configureMessageConverters(convertersBuilder -> {
 						HttpMessageConverters converters = convertersBuilder.build();
-						if (converters.iterator().hasNext()) {
-							this.converter = JsonConverterDelegate.of(converters);
+						if (converters.isEmpty()) {
+							converters = HttpMessageConverters.forClient().registerDefaults().build();
 						}
+						this.converter = JsonConverterDelegate.of(converters);
 					})
 					.build();
 		}