Use ghost user if user was not found (#29161) (#29169)

Backport #29161

models/issues/comment_list.go

@@ -225,6 +225,10 @@ func (comments CommentList) loadAssignees(ctx context.Context) error {
 
 	for _, comment := range comments {
 		comment.Assignee = assignees[comment.AssigneeID]
+		if comment.Assignee == nil {
+			comment.AssigneeID = user_model.GhostUserID
+			comment.Assignee = user_model.NewGhostUser()
+		}
 	}
 	return nil
 }