Annotate indexOf(..) and lastIndexOf(..) ListOperations with @nullable.

Closes spring-projects#2488

Author: jxblum

src/main/java/org/springframework/data/redis/core/ListOperations.java

@@ -388,6 +388,7 @@ default V move(K sourceKey, Direction from, K destinationKey, Direction to, Dura
 	 * @since 2.4
 	 * @see <a href="https://redis.io/commands/lpos">Redis Documentation: LPOS</a>
 	 */
+	@Nullable
 	Long indexOf(K key, V value);
 
 	/**
@@ -400,6 +401,7 @@ default V move(K sourceKey, Direction from, K destinationKey, Direction to, Dura
 	 * @since 2.4
 	 * @see <a href="https://redis.io/commands/lpos">Redis Documentation: LPOS</a>
 	 */
+	@Nullable
 	Long lastIndexOf(K key, V value);
 
 	/**