fix: executes pending ddls if region memtable is empty while scheduling next flush

[evenyag]

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What's changed and what's your intention?

This PR fixes an issue: If a region schedules itself to flush again but its memtable is empty, then it will return success directly without clearing the FlushStatus and executing pending DDL requests.

So an alter request may be blocked for a long time in this situation:

1. the region triggers a flush
2. the region wants to flush again, it adds the flush task to the pending flush list
3. the region receives an alter request and it adds the requests to the pending DDL list
4. the region doesn't receive any write request
5. the flush job is finished and the worker schedules the same region to flush
6. the region checks the memtable and marks the pending flush request as success immediately because the memtable is empty, but lefts the alter request in the list

There is a debug assertion in the scheduler to detect a similar case. This PR also adds two tests to cover this case in different level.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 98.48485% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.13%. Comparing base (e982d2e) to head (365af39).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4119      +/-   ##
==========================================
- Coverage   85.39%   85.13%   -0.26%     
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  Files         994      994              
  Lines      173994   174346     +352     
==========================================
- Hits       148580   148429     -151     
- Misses      25414    25917     +503     

[killme2008]

LGTM

[sunng87]

Fixes #4120