org-super-agenda-keep-order breaks for auto groups

Using `(setq org-super-agenda-keep-order t)`, introduced recently by !167, causes a backtrace with `:auto-category` or `:auto-parent`. I didn't test other `:auto-*` groups, but I'm guessing it's likely to be the case for all of them.

Example agenda view:
```elisp
(setq org-agenda-custom-commands
'(("nc" "Next: category" todo ""
((org-super-agenda-groups
`((:auto-category t)))))))
```

Resulting backtrace when attempting to view it via `C-c a n c` (some sensitive data replaced with ``):
```
Debugger entered--Lisp error: (wrong-type-argument buffer-or-string-p (:name "Category: work" :items ...))
org-entries-lessp((:name "Category: work" :items ...) (:name "Category: prof..." :items ...))
org-super-agenda--group-items((... ... ... ... ... ... ... ... ... ... ... ... ... ... ...))
org-super-agenda--filter-finalize-entries(#())
org-agenda-finalize-entries((... ... ... ... ... ... ... ... ... ... ... ... ... ... ...) todo)
org-todo-list("")
#f(compiled-function () #)()
funcall(#f(compiled-function () #))
(let ((org-super-agenda-groups '((:auto-category t)))) (funcall '#f(compiled-function () #)))
eval((let ((org-super-agenda-groups '((:auto-category t)))) (funcall '#f(compiled-function () #))))
org-agenda(nil)
funcall-interactively(org-agenda nil)
command-execute(org-agenda)
```

I don't think that this has anything to do with my org files or other config, but let me know if you need more info and I can provide it.

---

Context:
- No problems when `org-super-agenda-keep-order` is not enabled
- org-super-agenda version: current `master` HEAD - commit fb5e2ef277bc811a3b061106c99e4c47b6b86f80
- org version: current `bugfix` HEAD - commit http://git.savannah.gnu.org/cgit/emacs/org-mode.git/commit/?id=73875939a
- Emacs version: 28.0.50, `master` HEAD as of about a day ago - commit https://github.com/emacs-mirror/emacs/commit/9a5a35c5a6