As I mentioned elsewhere in the thread, most users don’t remember what they used when they created the account, particularly if it’s something they don’t use often. It’s also cumbersome to have to input that, especially if you bundle that with an optional password field.
That’s not to say you don’t have a point about leaking that information. Personally I’d be more concerned about leaking the fact that I have an account at all. If this is a concern for you, you are likely not inclined to use the likes of Google Auth or Facebook Auth. You’d be better off using a unique password for each service, store them in some sort of password manager, and rely on the default behavior treating “local account” and “no account” the same in terms of showing you the password field.
Maybe that’s not your preferred behavior, but it does allow you to keep that data private while simultaneously being easier to use for the SSO users.