core: skip scheduling tasks if JOBSERVICE_URL is not set

[liubin]

Thank you for contributing to Harbor!

Comprehensive Summary of your change

Skip scheduling tasks if JOBSERVICE_URL is not set, this can avoid printing much error logs for about 5 minutes until health check failed.

Issue being fixed

Fixes #(issue)

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[codecov]

Codecov Report

Merging #19045 (18c4e2f) into main (444404f) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #19045      +/-   ##
==========================================
- Coverage   67.41%   67.40%   -0.02%     
==========================================
  Files         993      993              
  Lines      108896   108896              
  Branches     2751     2751              
==========================================
- Hits        73413    73402      -11     
- Misses      31537    31544       +7     
- Partials     3946     3950       +4     

Flag	Coverage Δ
unittests	67.40% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

see 5 files with indirect coverage changes

[github-actions]

This PR is being marked stale due to a period of inactivty. If this PR is still relevant, please comment or remove the stale label. Otherwise, this PR will close in 30 days.

[github-actions]

This PR was closed because it has been stalled for 30 days with no activity. If this PR is still relevant, please re-open a new PR against main.