NIFI-7279 This closes #4160. Protect against NPE in RedisDistributedMapCacheClientService when value is null

Signed-off-by: Joe Witt <joewitt@apache.org>

nifi-nar-bundles/nifi-redis-bundle/nifi-redis-extensions/src/main/java/org/apache/nifi/redis/service/RedisDistributedMapCacheClientService.java

@@ -199,7 +199,7 @@ public class RedisDistributedMapCacheClientService extends AbstractControllerSer
         return withConnection(redisConnection -> {
             final byte[] k = serialize(key, keySerializer);
             final byte[] v = redisConnection.get(k);
-            return valueDeserializer.deserialize(v);
+            return v == null ? null : valueDeserializer.deserialize(v);
         });
     }
 

nifi-nar-bundles/nifi-redis-bundle/nifi-redis-extensions/src/test/java/org/apache/nifi/redis/service/ITRedisDistributedMapCacheClientService.java

@@ -168,6 +168,7 @@ public class ITRedisDistributedMapCacheClientService {
             final ByteArrayOutputStream out = new ByteArrayOutputStream();
             final Serializer<String> stringSerializer = new StringSerializer();
             final Deserializer<String> stringDeserializer = new StringDeserializer();
+            final Deserializer<String> stringDeserializerWithoutNullCheck = new StringDeserializerWithoutNullCheck();
 
             final AtomicDistributedMapCacheClient cacheClient = context.getProperty(REDIS_MAP_CACHE).asControllerService(AtomicDistributedMapCacheClient.class);
 
@@ -185,6 +186,10 @@ public class ITRedisDistributedMapCacheClientService {
                 final String retrievedValue = cacheClient.get(key, stringSerializer, stringDeserializer);
                 Assert.assertEquals(value, retrievedValue);
 
+                // verify get returns null for a key that doesn't exist
+                final String missingValue = cacheClient.get("does-not-exist", stringSerializer, stringDeserializerWithoutNullCheck);
+                Assert.assertEquals(null, missingValue);
+
                 // verify remove removes the entry and contains key returns false after
                 Assert.assertTrue(cacheClient.remove(key, stringSerializer));
                 Assert.assertFalse(cacheClient.containsKey(key, stringSerializer));
@@ -261,4 +266,11 @@ public class ITRedisDistributedMapCacheClientService {
             return input == null ? null : new String(input, StandardCharsets.UTF_8);
         }
     }
+
+    private static class StringDeserializerWithoutNullCheck implements Deserializer<String> {
+        @Override
+        public String deserialize(byte[] input) throws DeserializationException, IOException {
+            return new String(input, StandardCharsets.UTF_8);
+        }
+    }
 }