feat: allow empty db password

[WTIFS]

Please provide issue(s) of this PR:
Fixes #1236

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• Does not have any user-facing changes. This may include API changes, behavior changes, performance improvements, etc.

[polaris-test-bot]

2023-09-05 16:37:00: =========TEST START==========
2023-09-05 17:34:17: [STANDALONE VERSION]PR autotest FAILED! We executed 47 test cases and 43 were passed. You can see test report in https://polaris-opentest-1306863770.cos.ap-guangzhou.myqcloud.com/pr1502377927/sta/qta-report.html and https://polaris-opentest-1306863770.cos.ap-guangzhou.myqcloud.com/pr1502377927/sta/ratelimit/qta-report.html
2023-09-05 17:34:21: [CLUSTER VERSION] PR autotest FAILED! We executed 39 test cases and 35 were passed. You can see test report in https://polaris-opentest-1306863770.cos.ap-guangzhou.myqcloud.com/pr1502377927/clu/qta-report.html
2023-09-05 17:34:25: [KUBENETES VERSION]PR autotest FAILED! We executed 39 test cases and 36 were passed. You can see test report in https://polaris-opentest-1306863770.cos.ap-guangzhou.myqcloud.com/pr1502377927/k8s/qta-report.html

[codecov]

Codecov Report

Merging #1237 (82c6610) into main (eb3a9cc) will increase coverage by 0.09%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##             main    #1237      +/-   ##
==========================================
+ Coverage   55.78%   55.87%   +0.09%     
==========================================
  Files         179      179              
  Lines       32438    32442       +4     
==========================================
+ Hits        18094    18127      +33     
+ Misses      11987    11963      -24     
+ Partials     2357     2352       -5     

Files Changed	Coverage Δ
store/mysql/default.go	73.36% <40.00%> (-1.08%)	⬇️

... and 8 files with indirect coverage changes

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