fix create_snapshot

[Besroy]

m_snapshot_context is nullptr in the first create_snapshot, add check to avoid crash.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 64.09%. Comparing base (1746bcc) to head (d4751e3).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...ib/homestore_backend/replication_state_machine.cpp	0.00%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
+ Coverage   63.15%   64.09%   +0.93%     
==========================================
  Files          32       32              
  Lines        1900     1972      +72     
  Branches      204      210       +6     
==========================================
+ Hits         1200     1264      +64     
- Misses        600      603       +3     
- Partials      100      105       +5     

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[JacksonYao287]

LGTM