'make epub' got an unexpected filename when project name is not in English

Describe the bug

in conf.py, if project value is set to a non-english name, such as a CJK(Chinese,Japan,Korean) string, this value will be ignored and the making result of epub will be sphinx.epub or ----.epub or others that can't be predicted.
(I guess that depends on whether there were any ASCII chars in the project value. All non-ASCII chars will be lost, and if there is zero ASCII char left after that ignoring operation, the filename will be sphinx.epub).

But if I add an epub_basename value in CJK, the result epub filename will be set to this value as expectation.

How to Reproduce

$ make epub
$ cd build/epub
$ ls *.epub

Expected behavior

make epub

shoud get an epub file in build/epub/
with a correct filename same to project value in conf.py

Your project

non_en_prj_demo.zip

Screenshots

No response

OS

Archlinux

Python version

3.9.6

Sphinx version

4.1.1

Sphinx extensions

No response

Extra tools

No response

Additional context

conf.py

# Configuration file for the Sphinx documentation builder.
#
# This file only contains a selection of the most common options. For a full
# list see the documentation:
# https://www.sphinx-doc.org/en/master/usage/configuration.html

# -- Path setup --------------------------------------------------------------

# If extensions (or modules to document with autodoc) are in another directory,
# add these directories to sys.path here. If the directory is relative to the
# documentation root, use os.path.abspath to make it absolute, like shown here.
#
# import os
# import sys
# sys.path.insert(0, os.path.abspath('.'))


# -- Project information -----------------------------------------------------

project = '测试标题-テストタイトル-테스트 이름-Название теста'
copyright = '2021, abc'
author = 'abc'

# The full version, including alpha/beta/rc tags
release = '1'
version = '3' # to suppress the epub warning about version...

# -- General configuration ---------------------------------------------------

# Add any Sphinx extension module names here, as strings. They can be
# extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
# ones.
extensions = [
]

# Add any paths that contain templates here, relative to this directory.
templates_path = ['_templates']

# The language for content autogenerated by Sphinx. Refer to documentation
# for a list of supported languages.
#
# This is also used if you do content translation via gettext catalogs.
# Usually you set "language" from the command line for these cases.
language = 'zh_CN'

# List of patterns, relative to source directory, that match files and
# directories to ignore when looking for source files.
# This pattern also affects html_static_path and html_extra_path.
exclude_patterns = []


# -- Options for HTML output -------------------------------------------------

# The theme to use for HTML and HTML Help pages.  See the documentation for
# a list of builtin themes.
#
html_theme = 'alabaster'

# Add any paths that contain custom static files (such as style sheets) here,
# relative to this directory. They are copied after the builtin static files,
# so a file named "default.css" will overwrite the builtin "default.css".
html_static_path = ['_static']


#epub confs

# epub_basename = '测试标题-テストタイトル-테스트 이름-Название теста'

# if this epub_basename was set, the filename of epub will be '测试标题-テストタイトル-테스트 이름-Название теста.epub'
# if this epub_base name was not set, the file name will be `----.epub`

The Project Zip

non_en_prj_demo.zip

Start with the Sphinx EPUB build triggered by make epub, then trace how project and epub_basename from conf.py become the filename in build/epub. Reproduce the issue with the supplied non-English project and verify that the generated .epub filename preserves the configured project value when no explicit basename is set.