bug: Crash on VimEnter when using `mapleader = "<C-Space>"` and setting a mapping with `<C-Space>` instead of `<Leader>` (or defining a group)

Did you check docs and existing issues?

[X] I have read all the which-key.nvim docs
[X] I have searched the existing issues of which-key.nvim
[X] I have searched the existing issues of plugins related to this issue

Neovim version (nvim -v)

0.9.5

Operating system/version

Linux (NixOS)

Describe the bug

I recently changed my main <Leader> to <C-Space> and opening neovim now shows an error message. And when I disable enough things to avoid this error message, the which-key popup is not displayed on <C-Space>..

Steps To Reproduce

Run nvim -u repro.lua 😬

See error:

Error detected while processing VimEnter Autocommands for "*":
E5108: Error executing lua ...wip/.repro/plugins/which-key.nvim/lua/which-key/util.lua:128: {
  internal = { "<", "C", "-", "S", "p", "a", "c", "e", ">", "f", "o", "o" },
  keystr = "<Leader>foo",
  notation = { "<", "L", "e", "a", "d", "e", "r", ">", "f", "o", "o" }
}
stack traceback:
        [C]: in function 'error'
        ...wip/.repro/plugins/which-key.nvim/lua/which-key/util.lua:128: in function 'parse_keys'
        ....repro/plugins/which-key.nvim/lua/which-key/mappings.lua:183: in function <....repro/plugins/which-key.nvim/lua/which-key/mappings.lua:171>
        vim/shared.lua: in function 'parse'
        ...wip/.repro/plugins/which-key.nvim/lua/which-key/keys.lua:208: in function 'register'
        ...wip/.repro/plugins/which-key.nvim/lua/which-key/init.lua:94: in function 'load'
        [string ":lua"]:1: in main chunk

Replace (in the repro) the which-key config with config = true to remove the group (to continue with another similar bug)
Re-open nvim -u repro.lua

See the same error again, this time for the mapping:

Error detected while processing VimEnter Autocommands for "*":
E5108: Error executing lua ...wip/.repro/plugins/which-key.nvim/lua/which-key/util.lua:128: {
  internal = { "<", "C", "-", "S", "p", "a", "c", "e", ">" },
  keystr = "<leader>",
  notation = { "<", "l", "e", "a", "d", "e", "r", ">" }
}
stack traceback:
        [C]: in function 'error'
        ...wip/.repro/plugins/which-key.nvim/lua/which-key/util.lua:128: in function 'parse_keys'
        ...wip/.repro/plugins/which-key.nvim/lua/which-key/keys.lua:342: in function 'add_hooks'
        ...wip/.repro/plugins/which-key.nvim/lua/which-key/keys.lua:350: in function 'add_hooks'
        ...wip/.repro/plugins/which-key.nvim/lua/which-key/keys.lua:334: in function 'update'
        ...wip/.repro/plugins/which-key.nvim/lua/which-key/init.lua:96: in function 'load'
        [string ":lua"]:1: in main chunk

Replace <C-Space> with <Leader> in the keybind at the bottom of the repo (to continue with other similar bugs that might be related)
No more error messages on VimEnter
But there seems to be other issues that might be related: a. I can't use the <C-Space>i keybind at all in this repro (I don't see the msg being echo-ed) b. Typing <C-Space> and waiting for &timeoutlen does not show the which-key popup 🤔

Expected Behavior

No error on start, and working normally

Repro

-- DO NOT change the paths and don't remove the colorscheme
local root = vim.fn.fnamemodify("./.repro", ":p")

-- set stdpaths to use .repro
for _, name in ipairs({ "config", "data", "state", "cache" }) do
  vim.env[("XDG_%s_HOME"):format(name:upper())] = root .. "/" .. name
end

-- bootstrap lazy
local lazypath = root .. "/plugins/lazy.nvim"
if not vim.loop.fs_stat(lazypath) then
  vim.fn.system({ "git", "clone", "--filter=blob:none", "https://github.com/folke/lazy.nvim.git", lazypath, })
end
vim.opt.runtimepath:prepend(lazypath)

-- install plugins
local plugins = {
  "folke/tokyonight.nvim",
  {
    "folke/which-key.nvim",
    config = function()
      require"which-key".register { ["<Leader>foo"] = "+some-group" }
      -- CRASH because of this config
    end,
  },
  -- add any other plugins here
}
require("lazy").setup(plugins, {
  root = root .. "/plugins",
})

vim.cmd.colorscheme("tokyonight")
-- add anything else here

vim.g.mapleader = "<C-Space>"
vim.keymap.set("n", "<C-Space>i", "<cmd>echo 'hello from leader'<cr>")
-- CRASH because of this mapping

-- NOTE: does not error if I replace "<C-Space>i" with "<Leader>i"

folke / which-key.nvim

bug: Crash on VimEnter when using `mapleader = "<C-Space>"` and setting a mapping with `<C-Space>` instead of `<Leader>` (or defining a group) #587