Add lint against empty Union

Bug Report

From the documentation it indicates that Union must contain at least one item, however this doesn't seem to be implemented in mypy (and in fact, mypy seems to treat an empty union as NoReturn) -- the treating it as NoReturn is actually kinda cool, but probably not what was intended (?)

my twitch chat is telling me that pyright also considers Union[()] as NoReturn, maybe this just needs cpython documentation

To Reproduce

from typing import Union

def f() -> Union:
    print('hello hello')

def g() -> Union[()]:
    print('hello hello')

$ mypy --version
mypy 0.800
$ mypy t.py
t.py:3: error: Implicit return in function which does not return
t.py:6: error: Implicit return in function which does not return
Found 2 errors in 1 file (checked 1 source file)

Comienza con el reproductor de Union y Union[()] del issue e inspecciona cómo mypy informa de los retornos implícitos; no se identifica ningún archivo fuente ni ninguna prueba. Determina primero si la corrección prevista es un diagnóstico de mypy o un cambio en la documentación de CPython, y verifica después que el resultado elegido distinga una Union vacía del comportamiento actual de NoReturn.