Fix Option(None) in Annotated crashing with AttributeError

[bysiber]

Summary

Using Annotated[str, typer.Option(None)] crashes with AttributeError: 'NoneType' object has no attribute 'isidentifier'.

The Bug

In utils.py, when processing Annotated parameters, typer reinterprets Option's first positional argument from "default value" to "first param_decl name". The guard only checks parameter_info.default is not ..., which passes for None since None is not ... is True. The None value then gets cast(str, ...) and inserted into param_decls, and Click crashes when it tries to call .isidentifier() on it.

from typing import Annotated
import typer

app = typer.Typer()

@app.command()
def main(name: Annotated[str, typer.Option(None)] = "world"):
    print(f"Hello {name}")

app()
# AttributeError: 'NoneType' object has no attribute 'isidentifier'

The Fix

Added isinstance(parameter_info.default, str) to the guard so only actual strings are treated as param_decl names. Non-string values (like None) are left as default and handled by the normal default-value logic below.