Well, no Nobody was created for Riku because no soul was left to animate the body, and the body itself was possessed by Ansem(Xehanort's Heartless) the moment Riku's heart and soul were flung out to the realm of darkness. We can be sure that Riku's soul was accounted for because his consciousness came back to haunt XH and protect Kairi/Donald/Goofy from the Heartless.
Aside from that, I just want to point out that its not entirely impossible for Kairi to have a Heartless: Though her heart was with Sora, you have to remember that Riku still carried around her living (albeit nonfunctioning) body through Neverland and Hollow Bastion. Yet, even though her body did not fade and be reborn as a Nobody, Namine was still created. So its possible that this also created Xion, who could be a Heartless (explaining the black hair and darker skin) in the process... If Xion is a Heartless, this would explain why she is not in the Organization on any official basis (why OrgXIII is still XIII rather than XIV, why she is being kept in against her will, etc.) aside from Xemnas wanting her there. It would also be a bargaining chip against Namine, whether or not she knew that Xion was her other half...
And if the rule is that once a Heart separates from Body and Soul, creating a Nobody and a Heartless, doesn't that mean that when the body/soul and heart are accounted for, but a Nobody still is created, a Heartless would also be created? Though this brings something else into question: If this theory is right, then that would mean there are a potential 2 Kairi's running around (or were, anyways, before Xion ceased to exist and Namine rebonded with Kairi herself).
Exception that proves the rule!