Fix cache config needing to be provided
requested to merge 29453-panic-runtime-error-invalid-memory-address-or-nil-pointer-dereference into main
What does this MR do?
Ensures that a cache config isn't required for Runner to function.
Why was this MR needed?
Regression introduced in 15.6 resulted in a nil pointer panic if a cache config wasn't provided.
What's the best way to test this MR?
Manual QA:
Run any job with a missing [runner.cache] section in config.toml.
Integration test:
Run TestBuildCacheHelper test without the fix introduced to abstract.go.
What are the relevant issue numbers?
Closes #29453 (closed)