feat(event cache): don't auto-shrink if the sync returned 0 events

[bnjbvr]

Tests to be written.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.09%. Comparing base (3e3bff7) to head (5fe3fb4).
Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4719      +/-   ##
==========================================
+ Coverage   86.08%   86.09%   +0.01%     
==========================================
  Files         290      290              
  Lines       34072    34072              
==========================================
+ Hits        29331    29335       +4     
+ Misses       4741     4737       -4     

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[bnjbvr]

I am now convinced this is incorrect: if the sync included a gap but no events, we may be rendering events from before the gap in the timeline, but not the gap, leading to the impression that some events (those that would be returned by resolving the gap) have been lost by the app. Closing for now.