Do not throw if cache path is empty

[Meakk]

Describe your changes

On Android, caches are local to app, so it's the app responsibility to set the cache path using f3d::engine::setCachePath right after engine initialization.
Do not throw if the cache path cannot be retrieved using env vars.

Supersedes #2024

Issue ticket number and link if any

Checklist for finalizing the PR

• I have performed a self-review of my code
• I have added tests for new features and bugfixes
• I have added documentation for new features
• If it is a modifying the libf3d API, I have updated bindings
• If it is a modifying the .github/workflows/versions.json, I have updated docker_timestamp

Continuous integration

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[mwestphal]

There is surely a test to update.

Also, what is the behavior when there is no cache path then ?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.59%. Comparing base (d4869e9) to head (24e6fe3).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2752      +/-   ##
==========================================
- Coverage   96.61%   96.59%   -0.03%     
==========================================
  Files         142      142              
  Lines       12731    12736       +5     
==========================================
+ Hits        12300    12302       +2     
- Misses        431      434       +3     

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