bpo-24444: fix an error in argparse help when help for an option is blank

[akulakov]

I think it may be good to fix this because cmd-line help is more likely to be forgotten in testing and so a program may be distributed to users with one of these two bugs that break cmd line help and so make it unusable.

https://bugs.python.org/issue24444

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[miss-islington]

Thanks @akulakov for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10.
🐍🍒⛏🤖

[bedevere-bot]

GH-28931 is a backport of this pull request to the 3.10 branch.

[bedevere-bot]

GH-28932 is a backport of this pull request to the 3.9 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86-64 macOS 3.9 has failed when building commit fb72034.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/108/builds/301) and take a look at the build logs.
4. Check if the failure is related to this commit (fb72034) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/108/builds/301

Failed tests:

• test_importlib

Failed subtests:

• test_multiprocessing_pool_circular_import - test.test_importlib.test_threaded_import.ThreadedImportTests

Summary of the results of the build (if available):

== Tests result: FAILURE then FAILURE ==

406 tests OK.

10 slowest tests:

• test_concurrent_futures: 4 min 40 sec
• test_multiprocessing_spawn: 4 min 2 sec
• test_tokenize: 3 min 12 sec
• test_unparse: 2 min 56 sec
• test_multiprocessing_forkserver: 2 min 29 sec
• test_lib2to3: 2 min 3 sec
• test_asyncio: 1 min 51 sec
• test_capi: 1 min 48 sec
• test_unicodedata: 1 min 36 sec
• test_io: 1 min 3 sec

1 test failed:
test_importlib

18 tests skipped:
test_devpoll test_epoll test_gdb test_ioctl test_msilib
test_multiprocessing_fork test_ossaudiodev test_smtpnet test_spwd
test_ssl test_startfile test_tix test_tk test_ttk_guionly
test_winconsoleio test_winreg test_winsound test_zipfile64

1 re-run test:
test_importlib

Total duration: 28 min 2 sec

Click to see traceback logs

Traceback (most recent call last):
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/test/test_importlib/partial/pool_in_threads.py", line 9, in t
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/context.py", line 119, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild,
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/pool.py", line 212, in __init__
    self._repopulate_pool()
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/pool.py", line 303, in _repopulate_pool
    return self._repopulate_pool_static(self._ctx, self.Process,
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/pool.py", line 326, in _repopulate_pool_static
    w.start()
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/context.py", line 284, in _Popen
    return Popen(process_obj)
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/popen_fork.py", line 19, in __init__
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/popen_spawn_posix.py", line 42, in _launch
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/spawn.py", line 176, in get_preparation_data
OSError: [Errno 24] Too many open files
/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/resource_tracker.py:216: UserWarning: resource_tracker: There appear to be 120 leaked semaphore objects to clean up at shutdown
  warnings.warn('resource_tracker: There appear to be %d '
/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/resource_tracker.py:229: UserWarning: resource_tracker: '/mp-0rd_vgft': [Errno 2] No such file or directory
  warnings.warn('resource_tracker: %r: %s' % (name, e))
/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/resource_tracker.py:229: UserWarning: resource_tracker: '/mp-b3p9_4ru': [Errno 2] No such file or directory
  warnings.warn('resource_tracker: %r: %s' % (name, e))
/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/resource_tracker.py:229: UserWarning: resource_tracker: '/mp-n1jdmsuw': [Errno 2] No such file or directory
  warnings.warn('resource_tracker: %r: %s' % (name, e))
---


Traceback (most recent call last):
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/resource_tracker.py", line 201, in main
    cache[rtype].remove(name)
KeyError: '/psm_79d8031c'


Traceback (most recent call last):
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/multiprocessing/resource_tracker.py", line 201, in main
    cache[rtype].remove(name)
KeyError: '/psm_215f6f20'


Traceback (most recent call last):
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/test/test_importlib/test_threaded_import.py", line 258, in test_multiprocessing_pool_circular_import
    script_helper.assert_python_ok(fn)
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/test/support/script_helper.py", line 156, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/test/support/script_helper.py", line 142, in _assert_python
    res.fail(cmd_line)
  File "/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/test/support/script_helper.py", line 70, in fail
    raise AssertionError("Process return code is %d\n"
AssertionError: Process return code is 1
command line: ['/Users/buildbot/buildarea/3.9.billenstein-macos/build/python.exe', '-X', 'faulthandler', '-I', '/Users/buildbot/buildarea/3.9.billenstein-macos/build/Lib/test/test_importlib/partial/pool_in_threads.py']