BUG: ignored_filetypes not work for alpha-nvim

[adoyle-h]

Description

Even with ignored_filetypes = { 'alpha' }, the whitespace.nvim still highlight the whitespaces in alpha buffer.

With whitespace.nvim latest version and commit 21dfe34, it shows that

With commit 406cd69, it works well.

So I think this bug was introduced in 21dfe34

Reproduce

1. Create a file 'repro.lua'

-- **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)

vim.cmd.color 'elflord'
--------------------------------------------------------------------------------
local plugins = {

	{
    'goolord/alpha-nvim',
    config = function ()
			local logo = {
				[[                               __                ]],
				[[  ___     ___    ___   __  __ /\_\    ___ ___    ]],
				[[ / _ `\  / __`\ / __`\/\ \/\ \\/\ \  / __` __`\  ]],
				[[/\ \/\ \/\  __//\ \_\ \ \ \_/ |\ \ \/\ \/\ \/\ \ ]],
				[[\ \_\ \_\ \____\ \____/\ \___/  \ \_\ \_\ \_\ \_\]],
				[[ \/_/\/_/\/____/\/___/  \/__/    \/_/\/_/\/_/\/_/]],
			}

			require'alpha'.setup {
				layout =  { { type = 'text', val = logo , opts = { position = 'center'} } },
				opts = { noautocmd = true },
			}
    end
	},
	{
    'johnfrankmorgan/whitespace.nvim',
	# commit = '21dfe3478dced70327d2b08d0b84daec5ebf3610',  -- this commit has problem
    # commit = '406cd69216dd7847b0cb38486603a8ed5c4f8c77',  -- this commit has no problem
    config = function ()
        require('whitespace-nvim').setup({
            -- configuration options and their defaults

            -- `highlight` configures which highlight is used to display
            -- trailing whitespace
            highlight = 'DiffDelete',

            -- `ignored_filetypes` configures which filetypes to ignore when
            -- displaying trailing whitespace
            ignored_filetypes = { 'alpha' },

            -- `ignore_terminal` configures whether to ignore terminal buffers
            ignore_terminal = true,

            -- `return_cursor` configures if cursor should return to previous
            -- position after trimming whitespace
            return_cursor = true,
        })
    end
	},

}

require('lazy').setup(plugins, {
	root = root .. '/plugins',
})

2. nvim -u ./repro.lua it will auto download plugins on current directory.
3. set ft shows the filetype of current buffer is alpha