Prevent 500 errors when evaluating empty changelog configs

Joe Woodwardrequested to merge
fix/471717-empty-changelog-config-yaml into master

What does this MR do and why?

Prevent 500 errors when evaluating empty changelog configs

In 17.2 we fixed an issue when loading the changelog config yaml !156938 (merged)

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

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