Only update poster in issue/comment list if it has been loaded (#31216)

Previously, all posters were updated, even if they were not part of
posterMaps. In that case, a ghost user was erroneously inserted.

Fixes #31213.

models/issues/comment_list.go

@@ -32,7 +32,9 @@ func (comments CommentList) LoadPosters(ctx context.Context) error {
 	}
 
 	for _, comment := range comments {
-		comment.Poster = getPoster(comment.PosterID, posterMaps)
+		if comment.Poster == nil {
+			comment.Poster = getPoster(comment.PosterID, posterMaps)
+		}
 	}
 	return nil
 }