Fact is that a Heartless has to be involved in the birth process of a Nobody.
This has never been explicitly stated, the fandom has essentially just come to regard this as fact.
I really just think it has to do with misunderstandings.
The Ansem Reports never emphasized Heartless in the creation of Nobodies, ie:
"When the heart casts away the body, where does the body go?"
"But what of the soul and body left behind when the heart is lost? When the soul leaves the body, its vessel, life gives way to death, but what about when the heart leaves?"
The important part here is that the heart leaves the vessel. Not what happens to the heart afterward. Really, why would the heart becoming a Heartless have ANYTHING to do with the creation of a Nobody? Or put another way, why would the absence of a Heartless prevent a Nobody from being created? It doesn't make sense, there's no logical connection.
There are, as always, exceptions. Kairi didn't originally produce a Nobody, NOT because her heart didn't make a Heartless (though that itself shares the same reason), but because, as an entity of light, she could not leave the realm of light to be recreated as a Nobody.
So why didn't Riku make a Nobody? A similar reason. His body could not be recreated as a Nobody because it was being used by Ansem, he retained control of the vessel.
The underlying exception in these cases is that the body and soul cannot be used to create a Nobody, for whatever reason. It has been pretty clearly stated that a body and soul are needed in the creation of a Nobody.
But what about Namine?
"Naminé is Kairi's Nobody, but came into being via Sora's body and soul." Even I have debated in the past the meaning of this sentence, and argued against this, but I honestly think at this point that it simply means, yes, she used Sora's body and soul. Part of it, anyway. And this isn't at all hard to understand.
The body (and to a further extent, the soul, as what one can define as the "body's battery"), has been shown to be pretty unremarkable in this series. There's nothing very special about them. In fact, when normal humans die/lose their heart, they appear to disintegrate into light (suggesting that that is their material). People need to stop thinking from a western viewpoint that the body or soul are somehow important. They're not. In fact, Kairi essentially makes Sora new ones from her light when she saves him.
So, Sora never needed Roxas back because of his body and soul. He needed Roxas back because of his memories, and that's it.
In other words, I'm saying that Roxas and Namine simply split Sora's body and soul because there were two hearts that left his vessel (regardless of whether or not they made heartless), and only one body. People need to stop being hung up on the earlier statements in the Secret Ansem Reports when DiZ calls Namine a "non-being" that hasn't even become a true Nobody. That was a hypothesis. One that was debunked by another one in the report directly after it.
Yeah. I know I have made a few theories in the past and have spent a lot of time on this subject. I still like my most recent theory (which I conceded would not be the true answer, so it's more of an individual interpretation than a theory). And, of course, the problem is that the manner of Roxas and Namine's creations was conveyed very poorly by Nomura. But I think that many times we have simply overcomplicated things based on misunderstandings and rules devised in the fandom.
So, if Namine was using Sora's body and soul, was she technically Sora's Nobody too? Sure, depending on what you want to emphasize, though she was Kairi's above all else because she personified her. But that explains why she had control over Sora's memories, though did not actually claim them. Because she was born from Sora's body and soul (the containers of memories for the Nobody), but she took on Kairi's identity.
Why then did Namine have to return to Kairi? She didn't. She only did so because otherwise she herself would fade into darkness, it's not as though she was returning anything to Kairi (though it remains to be seen whether or not Kairi gained anything from this). Does it mean Kairi now has a piece of Sora's body and soul? No, because, again, that's western philosophy trying to invade your mind. Namine was made
from Sora's body and soul, but these things, broken down into their basic substance, are unremarkable and not very unique (only a heart makes them unique). It may have strengthened the connection between Sora and Kairi at best, but it is not as though Kairi literally has a piece of Sora in her.
So, going back to the original topic, does a Heartless have to be created? No. It simply must leave the vessel. 99 times out of 100 (or really much more than that), this entails that the heart will be lost to a Heartless, but it is not necessarily always the case.
Looking ahead, what does this suggest about the future?
For a hypothetical "Eraqus Nobody," nothing. Eraqus' heart wasn't simply leaving his body, Eraqus
died, which is something completely different. It looks the same, but don't let that fool you (they would opt for people to disintegrate into light/darkness rather than have a corpse). As opposed to just letting his heart go back to Kingdom Hearts (or perhaps become a Heartless, whatever happens to the heart after one dies), Eraqus, in his last moments, directed his heart into Terra's heart, probably to act as a guide. But his body and soul are separate and they're gone. There is no Nobody to be made, there is nothing to make it out of.
For Terranort (when he stabbed himself), again, nothing. We probably don't know the full extent of the effects, but no heart was shown leaving the vessel, and in any case, the vessel was still in control (like how Ansem was using Riku's body), so there's no way a Nobody was created.
And for Master Xehanort? Yeah, he might have a Nobody. His vessel was left behind, and that's all that matters. When I made a theory about MX potentially making a Nobody, I thought that I'd have to prove MX made a heartless when he entered Terra's vessel, but that's really not the case.
