pipeline upload failure if no steps are configured

[valkum]

If you have a watched path without any configured steps

plugins:
- monorepo-diff#v1.4.0:
    diff: ".buildkite/scripts/git-diff.sh main ${BUILDKITE_COMMIT}"
    watch:
      - path:
          - "some-oath/**"
        config:
          steps:
      - path:
          - "other-path/**"
        config:
          steps:
            - command: do-something

there is no error thrown that is helpful.
Instead, the plugin creates a pipeline that fails to upload. The pipeline looks like this:

Generated Pipeline:
steps:
    - {}

The resulting error is

FATA[2025-06-30T04:33:20-07:00] +++ failed to upload pipeline: command `buildkite-agent` failed: exit status 1

Do note that config is actually invalid, as at least label or group is missing when passing a step key. But monorepo-diff ignores this and emits an invalid pipeline for the agent to upload. But monorepo-diff ignores this and emits an invalid pipeline for the agent to upload.