Prevent failing gh actions runs when head_commit is missing
Prevent failing gh actions runs when head_commit is missing

In some cases, the `head_commit` object is not present in github actions
environments. I don't know exactly when this can happen, but adding a
null-check should help get things resolved. The CHANGE_URL env variable
is optional anyway, so no harm done when it's missing.

enduire/happo.io,enduire/happo.io