"Morning Sunshine" was not the first time Dean called Cas Sunshine.
In season 12, Dean called Cas “sunshine”.
However, this was not the first time Dean called Cas that.
In 7x21, Cas is talking with Hester whilst Dean makes an angel banishing sigil, he then says “Heads up, Sunshine.” Now, Dean wouldn’t give Hester a heads up, he’d give Cas a heads up because he didn’t want to banish Cas but he had to to banish Hester.
So yeah Dean has already called Cas “sunshine” before. Which, by the way, is adorable.