Fix import handling for filelock dependency

[bruAristimunha]

PR Description

Improve error handling by ensuring lock helpers skip reference counting when the optional filelock dependency is missing, preventing potential crashes during reads.

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

• All comments are resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• New contributors have been added to CITATION.cff
• PR description includes phrase "closes <#issue-number>"

[larsoner]

Can we add some test that would have caught this failure? Like a test that only runs if FileLock is not installed? Or monkey-patches to pretend like it's not installed or something...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.90%. Comparing base (51126eb) to head (9caaed5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1469      +/-   ##
==========================================
- Coverage   96.92%   96.90%   -0.02%     
==========================================
  Files          43       43              
  Lines       10003    10015      +12     
==========================================
+ Hits         9695     9705      +10     
- Misses        308      310       +2     

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[bruAristimunha]

mocked test done :)

[larsoner]

Marking for merge-when-green, thanks in advance @bruAristimunha !

[larsoner]

Appears to have fixed the failure observed in mne-tools/mne-nirs#647, thanks for the lightning quick fix @bruAristimunha !