Test for change to add conversion of defaultValue

This commit adds a test and polishing for a change in
AbstractNamedValueMethodArgumentResolver erroneously committed
with (unrelated) commit e57b942b.

If an argument becomes null after conversion and a default value is
applied, that default value should also pass through conversion.

Closes gh-31336

spring-web/src/main/java/org/springframework/web/method/annotation/AbstractNamedValueMethodArgumentResolver.java

@@ -274,9 +274,10 @@ public abstract class AbstractNamedValueMethodArgumentResolver implements Handle
 		return value;
 	}
 
+	@Nullable
 	private static Object convertIfNecessary(
 			MethodParameter parameter, NativeWebRequest webRequest, WebDataBinderFactory binderFactory,
-			NamedValueInfo namedValueInfo, Object arg) throws Exception {
+			NamedValueInfo namedValueInfo, @Nullable Object arg) throws Exception {
 
 		WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
 		try {