Found it 🙂
https://github.com/NodeBB/nodebb-theme-persona/blob/master/templates/partials/topic/post.tpl#L16
change that line to...
<a href="<!-- IF posts.user.userslug -->{config.relative_path}/user/{posts.user.userslug}<!-- ELSE -->#<!-- ENDIF posts.user.userslug -->" itemprop="author" data-username="{posts.user.username}" data-uid="{posts.user.uid}">{posts.user.fullname}</a>Full Name is an optional field so not everybody updates it in their profile. You might want to check if it's available before displaying it.
<a href="<!-- IF posts.user.userslug -->{config.relative_path}/user/{posts.user.userslug}<!-- ELSE -->#<!-- ENDIF posts.user.userslug -->" itemprop="author" data-username="{posts.user.username}" data-uid="{posts.user.uid}"><!-- IF posts.user.fullname -->{posts.user.fullname}<!-- ELSE -->{posts.user.username}<!-- ENDIF posts.user.fullname --></a>
