Unable to include empty docstring

[sbliven]

Empty docstrings are used by sphinx to indicate methods that should not be included in the documentation. However, as of 21.4b0 black reformats empty docstrings to contain a space.

To Reproduce:
Source:

def foo():
    """"""
    ...

gets reformatted to

def foo():
    """ """
    ...

Expected behavior I'm generally supportive of the style changes introduced in #1740. However, in this case black is changing the code in a meaningful way. I suggest that empty docstrings be ignored as a special case. I am agnostic on whether the empty docstring should be spelled "" or """""".

Environment:

• Version: 21.4b2
• OS and Python version: debian 10/Python 3.7.10

Does this bug also happen on master? Yes

[JelleZijlstra]

See #2150 for some similar previous discussion. I'm supportive of leaving empty docstrings alone to support this use case.

[sbliven]

See #2150 for some similar previous discussion. I'm supportive of leaving empty docstrings alone to support this use case.

If #2150 resolves in favor of reducing AST modifications that would certainly fix this bug. However a special case allowing empty docstrings would be sufficient for my use case.

[sethmlarson]

This is impacting the urllib3 documentation, we use an empty docstring to avoid having subclass methods from Python's http.client.HTTPConnection that aren't relevant to our users show up in our documentation generated via autodoc.

Reference: urllib3/urllib3#2239