Avoid failure on empty/missing cache

[bjester]

Summary

Observed that the Linting workflow (from pre-commit.yml) failed after merging the uv migration #5719

From the action's documentation:

By default, the action will fail if caching is enabled but there is nothing to upload (the uv cache directory does not exist). If you want to ignore this, set the ignore-nothing-to-cache input to true.

References

https://github.com/learningequality/studio/actions/runs/22630312994/job/65578292669

Reviewer guidance

Does the Linting workflow pass?

[rtibbles]

Does the Linting workflow pass?

yes