Nullability refinements and related polishing

spring-context/src/main/java/org/springframework/ui/ConcurrentModel.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -66,7 +66,8 @@ public class ConcurrentModel extends ConcurrentHashMap<String, Object> implement
 
 
 	@Override
-	public Object put(String key, Object value) {
+	@Nullable
+	public Object put(String key, @Nullable Object value) {
 		if (value != null) {
 			return super.put(key, value);
 		}