Fix incorrect http status not found behaviour in MySQL storage

[roger2hk]

#21

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 34.88%. Comparing base (46ec9c2) to head (71830c3).
Report is 48 commits behind head on main.

Files	Patch %	Lines
cmd/example-mysql/main.go	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   35.80%   34.88%   -0.93%     
==========================================
  Files          16       33      +17     
  Lines        1363     2775    +1412     
==========================================
+ Hits          488      968     +480     
- Misses        801     1699     +898     
- Partials       74      108      +34     

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