Fix: filesystem edge case in `ModelCheckpoint`

[schmidt-ai]

Fixes #17912.

[schmidt-ai]

@awaelchli I could take a stab at adding a test case if you'd prefer? Though I may need a little additional detail on your idea for it!

[codecov[bot]]

Codecov Report

Merging #18252 (8a760f2) into master (f52fedc) will decrease coverage by 35%. The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18252      +/-   ##
==========================================
- Coverage      84%      49%     -35%     
==========================================
  Files         439      431       -8     
  Lines       34484    34341     -143     
==========================================
- Hits        28877    16890   -11987     
- Misses       5607    17451   +11844     

[Borda]

@schmidt-ai @awaelchli how is it going here? :)

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