Use branch main and commit HEAD to trigger a contract_requiring_verification_published webhook when no provider versions exist

[canny]

Currently, if no provider versions exist, then no webhook can be triggered because the provider version number is required to trigger the build. We could use HEAD (most CI systems should respect that) as a placeholder in that situation, and default to the branch main (configurable).

https://pact.canny.io/admin/board/feature-requests/p/use-branch-main-and-commit-head-to-trigger-a-contractrequiringverificationpublis

[canny]

This issue has been linked to a Canny post: Use branch main and commit HEAD to trigger a contract_requiring_verification_published webhook when no provider versions exist 🎉