Empty config.initialData is overwritten with data from DOM

[scofalik]

📝 Provide detailed reproduction steps (if any)

To reproduce the error you need DOM element with some data and then create an editor on that DOM element and use initialData: '' in config.

In such configuration, I'd expect that the editor will be empty, but it actually is inited with the data from the DOM. That's because this code:

const initialData = config.initialData || getInitialData( sourceElementOrData );

return editor.data.init( initialData );

Which is incorrect. This code treats empty and unset initial data the same way.