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)

Start with the Union and Union[()] reproducer in the issue and inspect how mypy reports the implicit returns; no source file or test is identified. First determine whether the intended fix is a mypy diagnostic or a CPython documentation change, then verify that the chosen outcome distinguishes an empty Union from the current NoReturn behavior.