Fixes #806 with a more descriptive error when an empty config file is provided

Fixes #806 with a more descriptive error when an empty config file is provided
The `ConfigurationSourceProvider` isn't necessarily using a file-based backend for the configuration, it could as well be some remote network location or a database, so the message might be misleading.

Maybe change the message to "Configuration at $path must not be empty" to be more generic in the actual location where the configuration lives and more specific in the error message itself.