refactor: remove double checks of memtable size

[byte-sourcerer]

I hereby agree to the terms of the GreptimeDB CLA.

What's changed and what's your intention?

In the freeze_mutable function, the method memtables.mutable.is_empty() is checked twice - once in VersionControl::freeze_mutable and again in MemtableVersion::freeze_mutable. These check could be combined so the unwrap can be eliminated.

Checklist

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

[killme2008]

Good catch!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.10%. Comparing base (e142ca4) to head (69a1da0).

❗ Current head 69a1da0 differs from pull request most recent head e77c8b9

Please upload reports for the commit e77c8b9 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4117      +/-   ##
==========================================
- Coverage   85.40%   85.10%   -0.30%     
==========================================
  Files         994      994              
  Lines      174093   173988     -105     
==========================================
- Hits       148682   148073     -609     
- Misses      25411    25915     +504     

[v0y4g3r]

LGTM

[killme2008]

Thank you.