return null when not used in pipeline / transaction
class：org.redisson.spring.data.connection.RedissonConnection.java 
code line：882 
code segment：
<T> T write(byte[] key, Codec codec, RedisCommand<?> command, Object... params) {
	RFuture<T> f = executorService.writeAsync(key, codec, command, params);
	indexCommand(command);
	return sync(f);
}

bug：return null when not used in pipeline / transaction.

### Expected behavior
not return null when not used in pipeline / transaction.
### Actual behavior
return null when not used in pipeline / transaction.
### Steps to reproduce or test case
public Boolean setNx(String key, String value, long expireMillis) {
	return stringRedisTemplate.execute(new RedisCallback<Boolean>() {
		@Override
		public Boolean doInRedis(RedisConnection connection) throws DataAccessException {
			return connection.set(key.getBytes(), value.getBytes(), Expiration.milliseconds(expireMillis),
					SetOption.SET_IF_ABSENT);
		}
	}, true);
}
### Redis version
Redis-x64-3.0.500
### Redisson version
3.11.1
### Redisson configuration