fix: Skip executing stats for zero segment

[xiaocai2333]

issue: #40241

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.52%. Comparing base (a56b240) to head (875b189).
Report is 5 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #40448       +/-   ##
===========================================
+ Coverage   70.04%   80.52%   +10.47%     
===========================================
  Files         309     1474     +1165     
  Lines       27725   207407   +179682     
===========================================
+ Hits        19421   167010   +147589     
- Misses       8304    34332    +26028     
- Partials        0     6065     +6065     

Components	Coverage Δ
Client	79.69% <ø> (∅)
Core	70.03% <ø> (-0.02%)	⬇️
Go	82.27% <100.00%> (∅)

Files with missing lines	Coverage Δ
internal/datacoord/task_stats.go	91.43% <100.00%> (ø)

... and 1165 files with indirect coverage changes

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[czs007]

/approve
/lgtm

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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