Revert "only start with a timeline reset diff if the timeline isn't empty"

[ganfra]

Revert #6380 (but partially keep the updated comment)

• I've documented the public API changes in the appropriate changelog files
  (see Writing changelog entries).
• This PR was made with the help of AI.

Signed-off-by:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.96%. Comparing base (82c24a8) to head (0cfa07d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6599      +/-   ##
==========================================
- Coverage   89.96%   89.96%   -0.01%     
==========================================
  Files         382      382              
  Lines      107941   107941              
  Branches   107941   107941              
==========================================
- Hits        97112    97105       -7     
+ Misses       7168     7166       -2     
- Partials     3661     3670       +9     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

---

_{Comparing ganfra/timeline_initial_loading (0cfa07d) with main (82c24a8)}

[poljar]

To add a bit more clarity why this is getting reverted. The applications prefer the previos API shape so let us give the people what they want.