enhance: move GetVector_EmptySparseVector ut to growing test

[zhengbuqian]

in the latest version of Knowhere, only growing sparse index of IP metric will have raw data in index, thus no need to test GetVectorById of sealed index.

issue: #35853

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.07%. Comparing base (e501025) to head (558d02b).
Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #39234       +/-   ##
===========================================
+ Coverage   69.65%   81.07%   +11.42%     
===========================================
  Files         299     1404     +1105     
  Lines       26799   198260   +171461     
===========================================
+ Hits        18667   160747   +142080     
- Misses       8132    31862    +23730     
- Partials        0     5651     +5651     

Components	Coverage Δ
Client	79.50% <ø> (∅)
Core	69.65% <ø> (ø)
Go	82.99% <ø> (∅)

see 1105 files with indirect coverage changes

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[zhengbuqian]

/run-cpu-e2e

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