Fix issue in stubs where step decorators dont have any parameters

[talsperre]

If we have a decorator like @my_deco which does not have any parameters section in the docstring, then the generated stubs are wrong since it adds an empty Parameters section like the one shown below:

@typing.overload
def my_deco(f: typing.Callable[[FlowSpecDerived, StepFlag], None]) -> typing.Callable[[FlowSpecDerived, StepFlag], None]:
    """
    My function docstring
    
    Parameters
    ----------
    """

This causes issues when rendering the docs using mkdocs.