fix: nil ptr is used as nil interface in grpc client

[chyezh]

issue: #30715
pr: #30754

• Bug: Set nil struct pointer to describe nil interface.
  Panic with segment violation when calling method on this nil struct pointer.

[xiaofan-luan]

/lgtm
/approve

[mergify]

@chyezh ut workflow job failed, comment rerun ut can trigger the job again.

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chyezh, xiaofan-luan

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.16%. Comparing base (31f33f6) to head (4e27b5f).
Report is 3 commits behind head on 2.3.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.3   #30755      +/-   ##
==========================================
+ Coverage   82.11%   82.16%   +0.05%     
==========================================
  Files         841      841              
  Lines      121390   121400      +10     
==========================================
+ Hits        99677    99746      +69     
+ Misses      18468    18420      -48     
+ Partials     3245     3234      -11     

Files	Coverage Δ
internal/distributed/datacoord/client/client.go	96.09% <100.00%> (ø)
internal/distributed/datanode/client/client.go	90.90% <100.00%> (ø)
internal/distributed/indexnode/client/client.go	76.82% <100.00%> (ø)
internal/distributed/proxy/client/client.go	95.61% <100.00%> (ø)
internal/distributed/querycoord/client/client.go	93.38% <100.00%> (ø)
internal/distributed/querynode/client/client.go	88.29% <100.00%> (ø)
internal/distributed/rootcoord/client/client.go	94.29% <100.00%> (ø)

... and 24 files with indirect coverage changes

[jaime0815]

/lgtm