Fix check-deprecated-files hook to handle empty config file

The following discussion from !211295 should be addressed:

• @peterhegman started a discussion: (+1 comment)

  @smaglangit what do you think I should do with config/lint/deprecations.yml#L10 in this case? I tried changing to an empty array but that caused an error (though I might have the wrong YML syntax 🤔). Did you have an idea for how this file should look when there are no deprecated files in it?

  Also thanks for adding this linting, it was nice to run into it 🙂