bug inferring signatures from docstrings in stubgenc

Bug Report

When generating stubs from c modules, stubgen uses mypy.stubdoc to parse signatures from docstrings, if they are present, however, this does not work if the function takes no arguments

To Reproduce

Function with args:

>>> from mypy.stubdoc import infer_sig_from_docstring
>>> infer_sig_from_docstring('func(x) -> int', 'func')
[FunctionSig(name='func', args=[ArgSig(name='x', type=None, default=False)], ret_type='int')]

Function without args:

>>>: infer_sig_from_docstring('func() -> int', 'func')
[]  # <--- no signature!!!!

The fact that no signatures are returned means that the stubs that are generated look like this:

def func(*args, **kwargs) -> Any
    ...

But given that we have a docstring with a valid signature, the stubs should look like this:

def func() -> int
    ...

This seems like a bug. I'll make a PR to fix this when I have some time.

Start with mypy.stubdoc.infer_sig_from_docstring and reproduce the two examples from the issue, then trace how stubgen uses the returned signatures. Done means a no-argument docstring such as func() -> int yields a signature and stubgen produces the corresponding typed function stub instead of an *args/**kwargs fallback.