Refine null-safety in spring-web and spring-websocket

See gh-32475

spring-webflux/src/main/java/org/springframework/web/reactive/handler/WebFluxResponseStatusExceptionHandler.java

@@ -18,6 +18,7 @@ package org.springframework.web.reactive.handler;
 
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.http.HttpStatusCode;
+import org.springframework.lang.Nullable;
 import org.springframework.web.bind.annotation.ResponseStatus;
 import org.springframework.web.server.handler.ResponseStatusExceptionHandler;
 
@@ -38,6 +39,7 @@ import org.springframework.web.server.handler.ResponseStatusExceptionHandler;
 public class WebFluxResponseStatusExceptionHandler extends ResponseStatusExceptionHandler {
 
 	@Override
+	@Nullable
 	protected HttpStatusCode determineStatus(Throwable ex) {
 		HttpStatusCode statusCode = super.determineStatus(ex);
 		if (statusCode == null) {

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

@@ -66,6 +66,7 @@ public class RequestAttributeMethodArgumentResolver extends AbstractNamedValueSy
 	}
 
 	@Override
+	@Nullable
 	protected Object resolveNamedValue(String name, MethodParameter parameter, ServerWebExchange exchange) {
 		Object value = exchange.getAttribute(name);
 		ReactiveAdapter toAdapter = getAdapterRegistry().getAdapter(parameter.getParameterType());