Prevent 500 errors when evaluating empty changelog configs

In 17.2 we fixed an issue when loading the changelog config yaml
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/156938

This change caught errors from Psych when loading the YAML file. This
didn't take into account empty YAML files.

These changes check the output from safe_load to ensure there was
something loaded before trying to parse the config.

If nothing was loaded we return the default config.

Co-authored-by: @nav-j
Changelog: fixed