It would be great if `optimize_on_insert` was smart enough to discard empty blocks
```
create table empty (num Int8) engine=Memory;
CREATE MATERIALIZED VIEW empty_mv
(
    `num` Int8
)
ENGINE = SummingMergeTree
ORDER BY tuple() AS
SELECT num
FROM empty;

-- First insert

set optimize_on_insert=1;
insert into empty values (0);

-- 20 minutes break

set optimize_on_insert=0;
insert into empty values (0);

-- Created parts

SELECT
    event_type,
    event_time,
    table,
    part_name,
    rows
FROM system.part_log
PREWHERE (event_date = today()) AND (table LIKE '%empty%')
ORDER BY event_time ASC
FORMAT PrettyCompactMonoBlock

┌─event_type─┬──────────event_time─┬─table───────────┬─part_name─┬─rows─┐
│ NewPart    │ 2021-02-10 21:27:40 │ .inner.empty_mv │ all_1_1_0 │    0 │
│ RemovePart │ 2021-02-10 21:27:42 │ .inner.empty_mv │ all_1_1_0 │    0 │
│ NewPart    │ 2021-02-10 21:46:42 │ .inner.empty_mv │ all_2_2_0 │    1 │
└────────────┴─────────────────────┴─────────────────┴───────────┴──────┘
```
#include <Processors/Merges/Algorithms/ReplacingSortedAlgorithm.h>
#include <Processors/Merges/Algorithms/MergingSortedAlgorithm.h>
#include <Processors/Merges/Algorithms/CollapsingSortedAlgorithm.h>
#include <Processors/Merges/Algorithms/SummingSortedAlgorithm.h>
#include <Processors/Merges/Algorithms/AggregatingSortedAlgorithm.h>
#include <Processors/Merges/Algorithms/VersionedCollapsingAlgorithm.h>
#include <Processors/Merges/Algorithms/GraphiteRollupSortedAlgorithm.h>
namespace ProfileEvents
{
    extern const Event MergeTreeDataWriterBlocks;
    extern const Event MergeTreeDataWriterBlocksAlreadySorted;
    extern const Event MergeTreeDataWriterRows;
    extern const Event MergeTreeDataWriterUncompressedBytes;
    extern const Event MergeTreeDataWriterCompressedBytes;
}
namespace DB
{
namespace ErrorCodes
{
    extern const int LOGICAL_ERROR;
    extern const int TOO_MANY_PARTS;
}
namespace
{
void buildScatterSelector(
        const ColumnRawPtrs & columns,
        PODArray<size_t> & partition_num_to_first_row,
        IColumn::Selector & selector,
        size_t max_parts)
{