Hello. Thank you for contacting the Toolset support.
That's correct. You can not add the condition on WYSIWYG field.
However, possible workaround would be you can try it out if that helps, you can create a custom field group and add the custom field you want to display when WYSIWYG field is not empty to this custom field group.
And then you can use the hook "remove_meta_box" and "add_meta_box" to add/remove the custom field group meta box when WYSIWYG field is empty or not empty.