ConfigurationFilesCacheReloadTask prints warning for empty files

[JonasKunz]

When saving an empty File or a File with only whitespaces / comments, the ConfigurationFilesCacheReloadTask will show a warning ConfigurationFilesCacheReloadTask - Unable to load file with path ....

This happens because SnakeYAML returns null in this case, which we interpret as error.
This is wrong, we should only print a warning if SnakeYAML threw an exception.