Assume v3 store is always not nil

[serathius]

V3 version of #19695

[k8s-ci-robot]

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[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.19%. Comparing base (cc90010) to head (de5e4a5).

Files with missing lines	Patch %	Lines
server/etcdserver/api/membership/cluster.go	90.00%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/api/membership/storev2.go	50.53% <ø> (+5.34%)	⬆️
server/etcdserver/api/membership/cluster.go	89.12% <90.00%> (+0.42%)	⬆️

... and 62 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19698      +/-   ##
==========================================
+ Coverage   65.96%   68.19%   +2.22%     
==========================================
  Files         421      421              
  Lines       35858    35842      -16     
==========================================
+ Hits        23655    24442     +787     
+ Misses      10792     9960     -832     
- Partials     1411     1440      +29     

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[ahrtr]

similar comment as #19695 (comment)

[serathius]

/cc @ahrtr @fuweid @siyuanfoundation