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

• [x] add tests
• [x] document error

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #118 will not alter performance

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

Summary

✅ 6 untouched benchmarks

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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