[BUG] setup.py check -r results in TypeError when package has no long_description

setuptools version

setuptools==62.2.0

Python version

Python 3.7

OS

Windows 10

Additional environment information

No response

Description

Running the command python setup.py check -r with setuptools==62.2.0 installed causes a TypeError when the setup call in setup.py has not specified a long_description. This issue is not present in setuptools==62.1.0 .

It's kind of a silly problem, since check -r only seems to check the long_description anyways, but at least there should be a useful error message and not a cryptic TypeError. I'd probably just do nothing and let the check succeed if long_description is None. This worked before a7cfb56a7b1eaa95824d81a34e8e70b85d42f784 because the check was done on the internal string value "UNKNOWN", which turns out to be valid reStructuredText by itself.

Expected behavior

python setup.py check -r should succeed if there is no long_description.

How to Reproduce

1. Start with a very bare-bones pyproject with setup.py. Do not define a long_description.
2. pip install docutils (needed by -r option)
3. pip install setuptools==62.2.0
4. python setup.py check -r

Output

> type pyproject.toml
[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

> type setup.py
from setuptools import setup

setup(
    name='pkg',
    version='0.0.1',
    packages=['pkg'],
    install_requires=[],
)

> python -m pip install docutils
Collecting docutils
  Downloading docutils-0.18.1-py2.py3-none-any.whl (570 kB)
     ---------------------------------------- 570.0/570.0 kB 9.0 MB/s eta 0:00:00
Installing collected packages: docutils
Successfully installed docutils-0.18.1

> python -m pip install setuptools==62.2.0
Collecting setuptools==62.2.0
  Downloading setuptools-62.2.0-py3-none-any.whl (1.1 MB)
     ---------------------------------------- 1.1/1.1 MB 12.1 MB/s eta 0:00:00
Installing collected packages: setuptools
  Attempting uninstall: setuptools
    Found existing installation: setuptools 47.1.0
    Uninstalling setuptools-47.1.0:
      Successfully uninstalled setuptools-47.1.0
Successfully installed setuptools-62.2.0

> python setup.py check -r
running check
Traceback (most recent call last):
  File "setup.py", line 7, in <module>
    install_requires=[],
  File "C:\venv\lib\site-packages\setuptools\__init__.py", line 87, in setup
    return distutils.core.setup(**attrs)
  File "C:\venv\lib\site-packages\setuptools\_distutils\core.py", line 148, in setup
    return run_commands(dist)
  File "C:\venv\lib\site-packages\setuptools\_distutils\core.py", line 163, in run_commands
    dist.run_commands()
  File "C:\venv\lib\site-packages\setuptools\_distutils\dist.py", line 967, in run_commands
    self.run_command(cmd)
  File "C:\venv\lib\site-packages\setuptools\dist.py", line 1224, in run_command
    super().run_command(command)
  File "C:\venv\lib\site-packages\setuptools\_distutils\dist.py", line 986, in run_command
    cmd_obj.run()
  File "C:\venv\lib\site-packages\setuptools\_distutils\command\check.py", line 72, in run
    self.check_restructuredtext()
  File "C:\venv\lib\site-packages\setuptools\_distutils\command\check.py", line 102, in check_restructuredtext
    for warning in self._check_rst_data(data):
  File "C:\venv\lib\site-packages\setuptools\_distutils\command\check.py", line 130, in _check_rst_data
    parser.parse(data, document)
  File "C:\venv\lib\site-packages\docutils\parsers\rst\__init__.py", line 175, in parse
    convert_whitespace=True)
  File "C:\venv\lib\site-packages\docutils\statemachine.py", line 1512, in string2lines
    astring = whitespace.sub(' ', astring)
TypeError: expected string or bytes-like object

Reproduce the failure with the minimal setup.py and python setup.py check -r, then follow the traceback to the check_restructuredtext entry point and _check_rst_data. Done means the command succeeds when long_description is absent and the regression is covered by an appropriate test.