Consistent nullability for concurrent result

(cherry picked from commit b92877990d)

spring-web/src/main/java/org/springframework/web/context/request/async/WebAsyncManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -159,6 +159,7 @@ public final class WebAsyncManager {
 	 * concurrent handling raised one.
 	 * @see #clearConcurrentResult()
 	 */
+	@Nullable
 	public Object getConcurrentResult() {
 		return this.concurrentResult;
 	}