Only allow splatting iterables if function takes at least one positional arg

Bug Report

At runtime the following code snippet:

def f(*, x: int = 9, y:int = 8) -> None:
    pass

kwargs = {"x": 1, "y": 2}
f(*kwargs)

results in TypeError: f() takes 0 positional arguments but 2 were given. However, mypy does not report any issues.

To Reproduce

1. Execute python interpreter for the code snipped above.
2. Run mypy for the code snippet above.

Expected Behavior

Mypy should yield an error. For example pytype yields the following error:

Function f expects 0 arg(s), got 2 [wrong-arg-count]
         Expected: (*, x, y)
  Actually passed: (_, _)

Actual Behavior

Success: no issues found in 1 source file

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: None. Simply mypy <FILENAME>.
• Mypy configuration options from mypy.ini (and other config files): N/A (defaults)
• Python version used: 3.8.2
• Operating system and version: Mac OS Big Sur

Start by running the Python reproducer and the mypy command described in the issue, then trace how mypy checks a starred iterable passed to a function with only keyword-only parameters. Done means mypy reports an argument-count error for this example and the behavior is covered by a regression test.