Kingdom Hearts II Scar's nobody

Scar becomes an heartless in KH2, so a nobody should have been created as well, shouldn't he?

 

Yes
And it probably was. Somewhere.
Maybe he didn't end up creating a special Nobody, or maybe his Nobody will appear in a future game so they can bring Scar back for a boss fight whenever they bring the Pride Lands back

 

Well his hearts is strong enough to mantain his own mind as an heartless so I'm sure he created a special nobody

 

Not necessarily, Heartless come from how much Darkness there's in your heart, not how strong it is.
So, he may have had enough darkness to rival Ansem and got to keep his mind because, but still be weak, sort of like Riku on KH1, after he lost the Kingdom Key. I'm sure he had lots of darkness there, but his heart was too weak.

 

There's not a direct strength of heart to physical body correlation. Sora lost everything when he became a heartless, for example

 

which is hopefully never


I doubt Scar's Nobody will ever become a major plot point. It's probably best forgotten. There were some anomalies about Scar anyway, like his ghost.