Handle empty indicator when groupby is set

[droumis]

Fixes #119

Fix error when creating a new annotation from an empty annotator overlaid on an element and when .groupby has been set

• add tests
• document error

[codspeed-hq]

CodSpeed Performance Report

Merging #118 will not alter performance

_{Comparing handle_empty (8a44fc0) with main (0e8d4e8)}

Summary

✅ 6 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.85%. Comparing base (0e8d4e8) to head (8a44fc0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   86.77%   86.85%   +0.07%     
==========================================
  Files          25       25              
  Lines        2662     2677      +15     
==========================================
+ Hits         2310     2325      +15     
  Misses        352      352              

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