Currently, without an image or video, an Instagram Post/Message cannot be saved as a draft, so if the image or video isn't ready yet, but the text is, it cannot be started and saved for a later time without inserting a placeholder image.

This works similarly for any post that isn't valid to be published, for example a post that goes over a social network's character limit similarly cannot be saved. Since this would only be saved as a draft, it could be worth saving this way, with a message stating the reason it couldn't in its current state be published.