hack: correctly check for empty PULL_BASE_SHA
hack: correctly check for empty PULL_BASE_SHA

The existing comparison fails due to the 'nounset' option being on.
That failure masks the nicely formatted error message the if message is
trying to print. This correctly prints the intended error instead of
bash's default unset behavior.

jetstack/cert-manager,cert-manager/cert-manager,cert-manager/cert-manager,cert-manager/cert-manager,jetstack/cert-manager,jetstack-experimental/cert-manager,jetstack-experimental/cert-manager,jetstack/cert-manager,jetstack-experimental/cert-manager