Fix empty no_docstring_rgx not working correctly

[DanielNoord]

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Type of Changes

	Type
✓	🐛 Bug fix
✓	🔨 Refactoring

Description

Turns out we did not handle empty regex options correctly. The referenced issue was broken because whenever the no_docstring_rgx option was empty it would try to match re.compile("") which always matched and would thus never check a parameter.

The change to get_global_option() is necessary for it to actually return a Pattern[str] for those options. A problem I couldn't solve was that it is impossible (I think) to tell mypy that the option belongs to GLOBAL_OPTION_PATTERN and should therefore return a Pattern[str]. This is because you can't use isinstance on Literal and there is no good solution to do this any other way (using cast also doesn't work).

/CC @cdce8p Because you helped with #4978 and might have a solution for the mypy issue that I don't know about 😄

Closes #4136

[coveralls]

Pull Request Test Coverage Report for Build 1281877563

• 14 of 16 (87.5%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.004%) to 93.094%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pylint/utils/utils.py	13	15	86.67%

Totals
Change from base Build 1281362551:	-0.004%
Covered Lines:	13372
Relevant Lines:	14364

---

💛 - Coveralls

[DanielNoord]

❯ pip install -e .
Obtaining file:///Users/daniel/DocumentenLaptop/Programming/Github/pylint
Requirement already satisfied: platformdirs>=2.2.0 in /Users/daniel/.pyenv/versions/3.9.6/envs/pylint-3.9.6/lib/python3.9/site-packages (from pylint==2.11.2.dev0) (2.2.0)
Requirement already satisfied: astroid<2.9,>=2.8.0 in /Users/daniel/DocumentenLaptop/Programming/Github/astroid (from pylint==2.11.2.dev0) (2.8.0)
Requirement already satisfied: isort<6,>=4.2.5 in /Users/daniel/.pyenv/versions/3.9.6/envs/pylint-3.9.6/lib/python3.9/site-packages (from pylint==2.11.2.dev0) (5.9.3)
Requirement already satisfied: mccabe<0.7,>=0.6 in /Users/daniel/.pyenv/versions/3.9.6/envs/pylint-3.9.6/lib/python3.9/site-packages (from pylint==2.11.2.dev0) (0.6.1)
Requirement already satisfied: toml>=0.9.2 in /Users/daniel/.pyenv/versions/3.9.6/envs/pylint-3.9.6/lib/python3.9/site-packages (from pylint==2.11.2.dev0) (0.10.2)
Requirement already satisfied: typing-extensions>=3.10.0 in /Users/daniel/.pyenv/versions/3.9.6/envs/pylint-3.9.6/lib/python3.9/site-packages (from pylint==2.11.2.dev0) (3.10.0.0)
Requirement already satisfied: lazy_object_proxy>=1.4.0 in /Users/daniel/.pyenv/versions/3.9.6/envs/pylint-3.9.6/lib/python3.9/site-packages (from astroid<2.9,>=2.8.0->pylint==2.11.2.dev0) (1.6.0)
Requirement already satisfied: wrapt<1.13,>=1.11 in /Users/daniel/.pyenv/versions/3.9.6/envs/pylint-3.9.6/lib/python3.9/site-packages (from astroid<2.9,>=2.8.0->pylint==2.11.2.dev0) (1.12.1)
Requirement already satisfied: setuptools>=20.0 in /Users/daniel/.pyenv/versions/3.9.6/envs/pylint-3.9.6/lib/python3.9/site-packages (from astroid<2.9,>=2.8.0->pylint==2.11.2.dev0) (56.0.0)
Installing collected packages: pylint
  Attempting uninstall: pylint
    Found existing installation: pylint 2.11.2.dev0
    Uninstalling pylint-2.11.2.dev0:
      Successfully uninstalled pylint-2.11.2.dev0
  Running setup.py develop for pylint
Successfully installed pylint-2.11.2.dev0
WARNING: You are using pip version 21.2.2; however, version 21.2.4 is available.
You should consider upgrading via the '/Users/daniel/.pyenv/versions/3.9.6/envs/pylint-3.9.6/bin/python3.9 -m pip install --upgrade pip' command.
❯ pylint '/Users/daniel/DocumentenLaptop/Programming/Github/pylint/pylint/utils/utils.py'

--------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

Not sure what pre-commit is failing on. Passes locally when installing pylint from working directory.

[DanielNoord]

I wonder why the default value for no_docstring_rgx was ^_.
Perhaps there was a logic behind it, but users such as the one who initially reported this bug will never figure out that they need to add no_docstring_rgx= to their pylintrc for it to check all functions. I think changing it to be empty is more in line with what users will expect the default value of a regex pattern to be.

I would like confirmation on changing this default value before adding a commit that fixes the tests that fail because of the change.

Furthermore, it seems like the unittests are really prone to incorrectly testing settings. The change to "no_docstring_rgx": re.compile(""), in tests/extensions/test_check_docs.py shows that we might set settings to certain values with incorrect types and therefore passing/failing tests incorrectly. This is not something we can fix immediately but is something to keep in mind!

Edit: I can't reopen this because of force-pushing the branch. Opening a new PR.