Return single-element list with empty value when ReactiveHashOperations.hMGet(…) for a single key returns no value.

We now return a list containing a single empty KeyValue element when ReactiveHashOperations.hMGet(…) called for a single key returns no value.

Previously, the code used onErrorReturn(…) which returned the wrong value and suppressed errors.

Closes #2210

src/main/java/org/springframework/data/redis/connection/ReactiveHashCommands.java

@@ -283,7 +283,8 @@ public interface ReactiveHashCommands {
 	 * @see <a href="https://redis.io/commands/hget">Redis Documentation: HGET</a>
 	 */
 	default Mono<ByteBuffer> hGet(ByteBuffer key, ByteBuffer field) {
-		return hMGet(key, Collections.singletonList(field)).flatMapIterable(Function.identity()).next();
+		return hMGet(key, Collections.singletonList(field)).filter(it -> !it.contains(null))
+				.flatMapIterable(Function.identity()).next();
 	}
 
 	/**

src/main/java/org/springframework/data/redis/connection/lettuce/LettuceReactiveHashCommands.java

@@ -107,7 +107,7 @@ class LettuceReactiveHashCommands implements ReactiveHashCommands {
 			if (command.getFields().size() == 1) {
 				ByteBuffer key = command.getFields().iterator().next();
 				result = cmd.hget(command.getKey(), key.duplicate()).map(value -> KeyValue.fromNullable(key, value))
-						.map(Collections::singletonList).onErrorReturn(Collections.emptyList());
+						.defaultIfEmpty(KeyValue.empty(key)).map(Collections::singletonList);
 			} else {
 				result = cmd.hmget(command.getKey(), command.getFields().stream().toArray(ByteBuffer[]::new)).collectList();
 			}