DATAREDIS-824 - Fix potential NPE in DefaultReactiveHashOperations when deserializing values.

Avoid ByteBuffer creation for null values indicating absent keys in hash.

Original Pull Request: #340

Author: flisky

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

@@ -277,7 +277,7 @@ private List<HV> deserializeHashValues(List<ByteBuffer> source) {
 
 		List<HV> values = new ArrayList<>(source.size());
 		for (ByteBuffer byteBuffer : source) {
-			values.add(readHashValue(byteBuffer));
+			values.add(byteBuffer == null ? null : readHashValue(byteBuffer));
 		}
 		return values;
 	}

src/test/java/org/springframework/data/redis/core/DefaultReactiveHashOperationsIntegrationTests.java

@@ -178,6 +178,12 @@ public void multiGet() {
 		HK hashkey2 = hashKeyFactory.instance();
 		HV hashvalue2 = hashValueFactory.instance();
 
+		StepVerifier.create(hashOperations.multiGet(key, Arrays.asList(hashkey1, hashkey2))) //
+				.consumeNextWith(actual -> {
+					assertThat(actual).hasSize(2).containsSequence(null, null);
+				}) //
+				.verifyComplete();
+
 		putAll(key, hashkey1, hashvalue1, hashkey2, hashvalue2);
 
 		StepVerifier.create(hashOperations.multiGet(key, Arrays.asList(hashkey1, hashkey2))) //