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)

Inizia con il riproduttore di Union e Union[()] nell’issue e verifica come mypy segnala i ritorni impliciti; non sono indicati né un file sorgente né un test. Determina innanzitutto se la correzione prevista sia una diagnostica di mypy o una modifica alla documentazione di CPython, quindi verifica che il risultato scelto distingua una Union vuota dall’attuale comportamento di NoReturn.