Fix cache-read-only for scheduled jobs

The fix for #383 introduced a bug that caused the post-action to fail when
attemping to resolve `cache-read-only` for scheduled jobs.
This has been fixed with an explicit (rather than implicit) check for 'event.repository != null'.

Fixes #409

action.yml

@@ -18,7 +18,7 @@ inputs:
  When 'true', existing entries will be read from the cache but no entries will be written.
  By default this value is 'false' for workflows on the GitHub default branch and 'true' for workflows on other branches.
  required: false
- default: ${{ github.event.repository && github.ref_name != github.event.repository.default_branch }}
+ default: ${{ github.event.repository != null && github.ref_name != github.event.repository.default_branch }}
 
  cache-write-only:
  description: |