fix: handle error-state status contexts in --exclude flag

[pstibrany]

Summary

• StatusContext error state not excluded: The --exclude flag only checked for failure-state status contexts, but GitHub status contexts can also have state error (e.g. dev-policy-bot failing to load its policy file). Error-state contexts were never matched against the exclude list, so --exclude had no effect on them.
• Empty GITHUB_CI_EXCLUDE env var produces spurious entry: When GITHUB_CI_EXCLUDE is set to an empty string (e.g. from a Kubernetes container env var with an empty default), urfave/cli splits it into [""]. This is now filtered out so an empty env var behaves the same as unset.

Context

In Argo CD rollout workflows, the wait-for-github pr step is used to wait for PRs to be merged. The GITHUB_CI_EXCLUDE env var is the mechanism for per-deployment exclusion of known-flaky or irrelevant status checks. Both fixes are needed to make this work correctly:

1. Without the error-state fix, excluding a check like dev-policy-bot: master (which reports error, not failure) has no effect.
2. Without the empty-string fix, setting GITHUB_CI_EXCLUDE="" as a default in a shared template would add an empty entry to the exclude list.

Test plan

• Added test: error-state StatusContext is reported as failure (without exclude)
• Added test: error-state StatusContext is excluded when in exclude list
• All existing tests pass

[zerok]

Thank you 🙂

[pstibrany]

Thanks for review and adding to merge queue. I see that merging failed due to invalid commit title formatting. I've fixed it now.