After seeing a video pointing out that Pureblood Heartless don't have shadows, I went back to see footage of Sora as a Heartless and Ansem. The former has Sora have no shadow when he's a Shadow. During the fight with Riku with the Keyblade of heart, Riku's body still projects shadows. However, in the End of the World where Sora encounters Ansem posing as Riku, "Riku's" body doesn't appear to produce a shadow, and nether does Ansem himself, despite having the Heartless emblem on his chest.
The fact Ansem doesn't produce shadows normally yet still has the Heartless emblem on him is likely the reason why the Kingdom Hearts wiki doesn't make a statement as to whether he is an Emblem Heartless or a Pureblood Heartless. The infobox only says Heartless, where generally other Heartless characters have their infobox say if they're either an Emblem Heartless or a Pureblood Heartless.
So, do you think Ansem is a Pureblood Heartless, an Emblem Heartless, or some other kind of Heartless that doesn't fit into those groups?
Terra did not have a real nobody. Xehanort used his body as a vessel for his heart, so the body was still occupied.