Avoid null signals when resolving handler arguments

Prior to this commit, resolving an argument for a WebFlux controller
that's missing from the request and not required by the handler would
throw a NullPointerException in some cases.

This involves the conversion of the parameter (a `String` parameter type
might not trigger this behavior) and sending a `null` within a reactive
stream, which is illegal per the RS spec.

We now rely on a `Mono.justOrEmpty()` to handle those specific cases.

Issue: SPR-17050
(Cherry-picked from a7f97a1669)

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/AbstractNamedValueArgumentResolver.java

@@ -100,13 +100,13 @@ public abstract class AbstractNamedValueArgumentResolver extends HandlerMethodAr
 		Model model = bindingContext.getModel();
 
 		return resolveName(resolvedName.toString(), nestedParameter, exchange)
-				.map(arg -> {
+				.flatMap(arg -> {
 					if ("".equals(arg) && namedValueInfo.defaultValue != null) {
 						arg = resolveStringValue(namedValueInfo.defaultValue);
 					}
 					arg = applyConversion(arg, namedValueInfo, parameter, bindingContext, exchange);
 					handleResolvedValue(arg, namedValueInfo.name, parameter, model, exchange);
-					return arg;
+					return Mono.justOrEmpty(arg);
 				})
 				.switchIfEmpty(getDefaultValue(
 						namedValueInfo, parameter, bindingContext, model, exchange));