fix: default `package-name` to `undefined` (#459)
fix: default `package-name` to `undefined` (#459)

This allows for release-please to fallback to the deault value since it
uses the nullish coalescing operator for that check.

Fixes #457

Co-authored-by: Benjamin E. Coe <69440dae14aaa9fe2429b324bfce03449d09e1db@google.com>
google-github-actions/release-please-action