Fix NPE when manually flushing a batch

When manually flushing a batch with a set size, we could potentially run into a NPE if the command queue is empty when flush() is called.

src/main/java/io/lettuce/core/dynamic/SimpleBatcher.java

@@ -148,12 +148,12 @@ private List<RedisCommand<Object, Object, Object>> prepareForceFlush() {
 
         List<RedisCommand<Object, Object, Object>> batch = new ArrayList<>(Math.max(batchSize, 10));
 
-        do {
+        while (!queue.isEmpty()) {
+
             RedisCommand<Object, Object, Object> poll = queue.poll();
 
-            assert poll != null;
             batch.add(poll);
-        } while (!queue.isEmpty());
+        }
 
         return batch;
     }
@@ -166,7 +166,6 @@ private List<RedisCommand<Object, Object, Object>> prepareDefaultFlush(int consu
 
             RedisCommand<Object, Object, Object> poll = queue.poll();
 
-            assert poll != null;
             batch.add(poll);
         }
 

src/test/java/io/lettuce/core/dynamic/RedisCommandsBatchingIntegrationTests.java

@@ -99,6 +99,16 @@ void selectiveBatchingShouldHandleErrors() {
         }
     }
 
+    @Test
+    void shouldNotCrashWhenFlushCalledWithEmptyQueue() {
+
+        RedisCommandFactory factory = new RedisCommandFactory(redis.getStatefulConnection());
+
+        SelectiveBatchingWithSize api = factory.getCommands(SelectiveBatchingWithSize.class);
+
+        api.flush();
+    }
+
     @Test
     void shouldExecuteBatchingSynchronously() {
 
@@ -215,4 +225,11 @@ static interface SelectiveBatching extends Commands, BatchExecutor {
 
     }
 
+    @BatchSize(5)
+    interface SelectiveBatchingWithSize extends Commands, BatchExecutor {
+
+        void set(String key, String value);
+
+    }
+
 }