ValueError when using griffe as a docstring parser

[rudolfbyker]

Describe the bug

When using Griffe as a docstring-parsing library according to https://mkdocstrings.github.io/griffe/usage/#using-griffe-as-a-docstring-parsing-library , we sometimes get ValueError at

https://github.com/mkdocstrings/griffe/blob/master/src/griffe/dataclasses.py#L497

... which is caused by the warning method

https://github.com/mkdocstrings/griffe/blob/master/src/griffe/docstrings/utils.py#L37

... which is called from

https://github.com/mkdocstrings/griffe/blob/master/src/griffe/docstrings/google.py#L189

... when an annotation is missing.

To Reproduce

Run this:

from griffe.dataclasses import Docstring, Function, Parameters, Parameter
from griffe.docstrings.google import parse

if __name__ == "__main__":
    parsed_docstring = parse(
        Docstring(
            value="""\
A function.

Args:
    a: Whatever

Returns:
    Nothing.
""",
            lineno=1,
            parent=Function(
                name="foo",
                parameters=Parameters(
                    Parameter(
                        name="a",
                        annotation=None,
                        default="5",
                    )
                ),
            ),
        )
    )

Expected behavior
No error, just the usual warning. The warning need not contain the file path if there is no file path.

System (please complete the following information):

• griffe version: 0.13.0
• Python version: 3.7
• OS: Windows

Additional context
Add any other context about the problem here.

[rudolfbyker]

Another example, from the docs, but without annotation:

from griffe.dataclasses import Docstring, Function, Parameters, Parameter, ParameterKind
from griffe.docstrings.parsers import Parser, parse

function = Function(
    "func",
    parameters=Parameters(
        Parameter("param1", annotation=None, kind=ParameterKind.positional_or_keyword),  # I only changed this line
        Parameter("param2", annotation="int", kind=ParameterKind.keyword_only),
    ),
)
text = """
Hello I'm a docstring!

Parameters:
    param1: Description.
    param2: Description.
"""
docstring = Docstring(text, lineno=1, parent=function)
parsed = parse(docstring, Parser.google)

[pawamoy]

Thanks for the report! This is fixed in master (will push and release soon).