I can't find it anywhere, only country I can find with an Empire rank is Byzantium.
Here is the note saying which countries can be granted with an empire title via decision: http://www.eu4wiki.com/Government#Government_rank. The only starting empire in 1444 is Byzantium.
Update: looks like I was wrong. Ming also starts am empire, but it has it's own issues (50% autonomy, celestial empire government, mandate of heaven etc)