Fix missing null return type for Generator::current() & Generator::key()

[tscni]

For some reason #1280 changed the return type of Generator::current() from TYield|null to TYield, which is not correct.
Similarly, Generator::key() can return null as well.

For reference: https://3v4l.org/aSMe8

$generator = (function () {
    yield 0;
})();

var_dump($generator->current()); // 0
var_dump($generator->key()); // 0

$generator->next();

var_dump($generator->current()); // null
var_dump($generator->key()); // null