exit: null: numeric argument required

[tgolsson]

Describe the bug

Sometimes, the command hook fails to set a valid exit code on exit, due to being unable to retrieve the job exit code. There's a check above; but it seems like the exit code has the actual value "null", i.e., not empty string.

This causes the build step to fail, when it might otherwise be recoverable (from a CI standpoint).

For example:

Warning: init container failed with exit code null, this usually indicates plugin misconfiguration or infrastructure failure
/buildkite-data/plugins/github-com-EmbarkStudios-k8s-buildkite-plugin-v1-3-1/hooks/command: line 261: exit: null: numeric argument required

To Reproduce

No known reproduction.

Expected behavior

Preferably we don't retrieve a "null" error code.