feat(event cache): don't replace a gap chunk by an empty items chunks

[bnjbvr]

It was simpler to add a new function rather than trying to fiddle with replace_gap_at, especially in terms of the return type of this function. After a few back-and-forth, they're close enough that we could probably merge them together, if you have strong opinions about it.

This was the only place where we'd create an empty chunk, so with this, the storage doesn't keep empty chunks anymore \o/

Part of #3280.

[codecov]

Codecov Report

Attention: Patch coverage is 87.09677% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.41%. Comparing base (5d8ad3a) to head (1a5d217).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-common/src/linked_chunk/mod.rs	85.18%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4423      +/-   ##
==========================================
+ Coverage   85.39%   85.41%   +0.02%     
==========================================
  Files         283      283              
  Lines       31473    31490      +17     
==========================================
+ Hits        26876    26898      +22     
+ Misses       4597     4592       -5     

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[Hywan]

Looks good, thanks! I think it's fine to keep remove_gap_at and replace_gap_at separated.