Annotate Object#equals parameter with @nullable

See gh-23093

Co-authored-by: Sebastien Deleuze <sdeleuze@pivotal.io>

spring-core/src/main/java/org/springframework/util/LinkedCaseInsensitiveMap.java

@@ -273,7 +273,7 @@ public class LinkedCaseInsensitiveMap<V> implements Map<String, V>, Serializable
 	}
 
 	@Override
-	public boolean equals(Object obj) {
+	public boolean equals(@Nullable Object obj) {
 		return this.targetMap.equals(obj);
 	}