allow empty sparse row

[zhengbuqian]

by not adding zeros to the inverted index, while still adding to the raw data.

added unit tests for searching/retrieving.

issue: #687
/kind improvement

[sre-ci-robot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.30%. Comparing base (3c46f4c) to head (1f8c67e).
Report is 119 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##           main     #704       +/-   ##
=========================================
+ Coverage      0   72.30%   +72.30%     
=========================================
  Files         0       69       +69     
  Lines         0     4868     +4868     
=========================================
+ Hits          0     3520     +3520     
- Misses        0     1348     +1348     

see 69 files with indirect coverage changes

[alexanderguzhva]

/lgtm