The thing is, before Xehanort's experiments, there were no Heartless in the Realm of Light.
If any heart had been consumed by the darkness and formed a Heartless, then there would be Heartless in the Realm of Light.
Which means there's something we're missing.
Master Xehanort released his heart when he stole Terra's body. He released Ven's heart when he split it.
A heartless was formed in neither of those cases, even though hearts exposed to darkness are known to turn into Heartless (see: Sora's sacrifice in Hollow Bastion).
The question I have is this: did something happen in the process of Xehanort's experiments that made it possible for a Realm of Light heart to transform into a Heartless via excess darkness?
In any case, though, if there's no Realm of Light Heartless, there's also no Heartless made from Realm of Light beings. And, if Heartless are not made from Realm of Light beings, their creation would not involve the disappearance of a body, and hence a Nobody would not have a reason to exist.
As it stands, Heartless are natural existences gone awry. Nobodies were never meant to exist.