Document that MapPropertySource should not contain null values

Closes gh-25142

spring-core/src/main/java/org/springframework/core/env/MapPropertySource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -23,6 +23,8 @@ import org.springframework.util.StringUtils;
 
 /**
  * {@link PropertySource} that reads keys and values from a {@code Map} object.
+ * The underlying map should not contain any {@code null} values in order to
+ * comply with {@link #getProperty} and {@link #containsProperty} semantics.
  *
  * @author Chris Beams
  * @author Juergen Hoeller
@@ -31,6 +33,12 @@ import org.springframework.util.StringUtils;
  */
 public class MapPropertySource extends EnumerablePropertySource<Map<String, Object>> {
 
+	/**
+	 * Create a new {@code MapPropertySource} with the given name and {@code Map}.
+	 * @param name the associated name
+	 * @param source the Map source (without {@code null} values in order to get
+	 * consistent {@link #getProperty} and {@link #containsProperty} behavior)
+	 */
 	public MapPropertySource(String name, Map<String, Object> source) {
 		super(name, source);
 	}