EMPTY_MOBILEDOC should not be set if html prop is passed.

[herregroen]

The mobiledoc-kit editor will use mobiledoc if available but otherwise parse HTML if that is available.

The react component will always pass EMPTY_MOBILEDOC if no mobiledoc prop is given meaning that the mobiledoc-kit editor will always have mobiledoc available and thus never parse HTML.

This makes the html prop unusable as there's no situation where mobiledoc-kit will use it's value.