Setting empty receipts in case of a newly added stack

[pacostas]

Summary

When adding a new stack, receipts, and hash code do not exist on previous release assets. In that case, we should handle the download and find asset actions to not error but instead set them to empty values.

Use Cases

Related issues

• https://github.com/paketo-community/ubi-base-stack/issues/13 (close after this PR has been merged)

Checklist

• [x] I have viewed, signed, and submitted the Contributor License Agreement.
• [x] I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• [x] I have added an integration test, if necessary.
• [x] I have reviewed the styleguide for guidance on my code quality.
• [x] I'm happy with the commit history on this PR (I have rebased/squashed as needed).