Fix greylist issue check cves justifier

This MR changes pipeline_wl_compare.py and justifier.py so they attempt to use/load the hardening_manifest.yaml for source and base images before attempting to load the greylist. This prevents new projects, which just have a hardening_manifest.yaml and no .greylist file from failing the lint, csv-output, and check-cves stages.

Relevant pipelines:

https://repo1.dso.mil/dsop/opensource/pipeline-test-project/grafana/-/pipelines/121455

https://repo1.dso.mil/dsop/ivx/yet-another-cloudwatch-exporter/-/jobs/1330091

https://repo1.dso.mil/dsop/opensource/pipeline-test-project/grafana/-/pipelines/121445