Initializing Paginator with show_menu=False throws an undocumented error when all the pages are PageGroup objects

Minimal reproducible example:

from discord.ext.pages import Paginator, PageGroup
Paginator(pages=[PageGroup(["a","b"], "label")])

Resulting error:

  File "c:\temp\bot\src\help.py", line 49, in get_help_paginator
     Paginator(pages=[PageGroup(["a","b"], "label")])
  File "C:\temp\bot\.venv\Lib\site-packages\discord\ext\pages\pagination.py", line 412, in __init__
    if all(isinstance(pg, PageGroup) for pg in pages):
                                               ^^^^^
TypeError: 'NoneType' object is not iterable

Caused by this line

If show_menu is false (dafault argument for the Paginator), line 422 tries to iterate over the None assigned in line 421.

Expected behavior should be following, based on the Paginator docstring.
If a list of :class:`PageGroup` objects is provided and `show_menu` is ``False``, only the first page group will be displayed.