Empty settings.yaml required for suborg and repo level settings to apply

## Problem Description
IDK if this is a "bug" or a request for updated documentation or a real feature request.

Running the bot without having a `settings.yaml` file causes the repo and suborg level settings to not apply. Reading the doc and the code, it looks like we should be able to specify suborg and repo level settings without an org level `settings.yaml`. Either the README should be updated to specifically call out that the `settings.yaml` is required, even if it is empty, or the bot should handle no `settings.yaml` the same way it handles a file with no content.

If we add an empty `settings.yaml` file the suborg and repo level changes are picked up and applied.

### What is actually happening
Without an org level `settings.yaml` the suborg and repo level settings do not apply.

### What is the expected behavior
Not having an org level `settings.yaml` should behave the same as having an empty org level `settings.yaml`

### Error output, if available

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## Context

### Are you using the hosted instance of probot/settings or running your own?
Own instance deployed in Azure.

### If running your own instance, are you using it with github.com or GitHub Enterprise?
With a GHEC org.

#### Version of probot/settings
`2.0.21-rc.8`

We did set the bot to read from `.github` instead of the `admin` repo.

#### Version of GitHub Enterprise