Buildkite generator generates no steps if only GitHub action YAML file is edited in PR, failing CI

Created by: felixfbecker

See this build: https://buildkite.com/sourcegraph/sourcegraph/builds/165143#01826492-e910-4029-9fbc-81e930402936

And the PR: https://github.com/sourcegraph/sourcegraph/pull/39885

Only a YAML file for a GitHub action was edited. Expected would be that this results in an empty CI run (or a minimal one).

Instead, the build always fails because the pipeline generator generates no steps property at all, which is required.