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 96.66%. Comparing base (d09e08b) to head (02cb8d9).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2752   +/-   ##
=======================================
  Coverage   96.66%   96.66%           
=======================================
  Files         142      142           
  Lines       12918    12923    +5     
=======================================
+ Hits        12487    12492    +5     
  Misses        431      431           

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[mwestphal]

A question