fix: streaming service stat manager crash when pv stat is empty

[chyezh]

issue: #33285

[mergify]

@chyezh E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[chyezh]

/run-cpu-e2e

[mergify]

@chyezh E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[chyezh]

rerun ut

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.

Project coverage is 72.47%. Comparing base (f5b4a97) to head (ba717e2).
Report is 4 commits behind head on master.

Files	Patch %	Lines
...er/wal/interceptors/segment/stats/stats_manager.go	69.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #35439      +/-   ##
==========================================
- Coverage   81.54%   72.47%   -9.08%     
==========================================
  Files        1225     1225              
  Lines      143876   143882       +6     
==========================================
- Hits       117330   104274   -13056     
- Misses      22106    35177   +13071     
+ Partials     4440     4431       -9     

Files	Coverage Δ
...er/wal/interceptors/segment/stats/stats_manager.go	94.93% <69.23%> (-5.07%)	⬇️

... and 245 files with indirect coverage changes

[jaime0815]

/lgtm

[sre-ci-robot]

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