enhance: Add nilness linter and fix some small issues

[congqixia]

Add nilness for govet linter and fixed some detected issues

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.01%. Comparing base (fb870d2) to head (625290d).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #34049      +/-   ##
==========================================
+ Coverage   81.00%   81.01%   +0.01%     
==========================================
  Files        1067     1067              
  Lines      136726   136712      -14     
==========================================
+ Hits       110759   110764       +5     
+ Misses      21754    21738      -16     
+ Partials     4213     4210       -3     

Files	Coverage Δ
internal/indexnode/task_analyze.go	14.78% <ø> (+1.07%)	⬆️
internal/proxy/simple_rate_limiter.go	83.50% <100.00%> (ø)
internal/storage/azure_object_storage.go	89.14% <ø> (-0.41%)	⬇️

... and 25 files with indirect coverage changes

[XuanYang-cn]

/lgtm