fix the judgment of checking the vector field

[lizc2003]

[sre-ci-robot]

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[sre-ci-robot]

Welcome @lizc2003! It looks like this is your first PR to milvus-io/milvus-sdk-go 🎉

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (40fea39) 87.35% compared to head (f1d2199) 87.35%. Report is 104 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #442   +/-   ##
=======================================
  Coverage   87.35%   87.35%           
=======================================
  Files          31       31           
  Lines        2878     2879    +1     
=======================================
+ Hits         2514     2515    +1     
  Misses        274      274           
  Partials       90       90           

Files	Coverage Δ
client/client_grpc_index.go	78.12% <50.00%> (+0.17%)	:arrow_up: