When it's not possible to save empty replies - the warning ' Post has no content. Content is required to save a post.' will be displayed, there are ways to save blank responses.

1. Click to edit a reply - remove the content.
2. Blanked out replies can be saved.
3. Click to create a reply - add Comment from VE options - it can be saved , but nothing will be displayed to a user.

See also T119349: Creating empty summaries should not be allowed