**Describe the bug**
```
SELECT number % 100 AS k, sumArray(emptyArrayUInt8()) AS v FROM numbers(10) GROUP BY k
```
Fails under ASan.

scheduleThreadForNextBucket();
                    if (it->second)
                    {
                        res.swap(it->second);
                        break;
                    }
                    else if (current_bucket_num >= NUM_BUCKETS)
                        break;
                }
                parallel_merge_data->condvar.wait(lock);
            }
            return res;
        }
    }
private:
    const Aggregator & aggregator;
    ManyAggregatedDataVariants data;
    bool final;
    size_t threads;
    Int32 current_bucket_num = -1;
    Int32 max_scheduled_bucket_num = -1;
    static constexpr Int32 NUM_BUCKETS = 256;
    struct ParallelMergeData
    {
        std::map<Int32, Block> ready_blocks;
        std::exception_ptr exception;
        std::mutex mutex;
        std::condition_variable condvar;
        ThreadPool pool;
        explicit ParallelMergeData(size_t threads_) : pool(threads_) {}