Just noticed that if I create a question with only an image in the question text, the Quiz editing interface also displays the [Empty question text] warning. I have to agree that an image is not "text", but on the other hand we may argue that since it contains an image, the "question text" field is not empty.
Would you consider removing that [Empty question text] message if the question text field contains just anything (text, image, media, etc.) i.e. it is not empty?