Description:
------------
foreach calls next() on a generator, which for some reason triggers send(null) on the generator.

All previous versions are also affected:
https://3v4l.org/bfg64#v560

First reported here:
https://stackoverflow.com/q/37817315

Test script:
---------------
$generator = (function () {
    for ($i = 0; $i < 5; $i += 1) {
        $answer = (yield $i * 2);
        echo "received: $answer\n";
    }
})();

foreach ($generator as $x) {
    echo "sending $x\n";
    $generator->send($x);
}

Expected result:
----------------
sending 0
received: 0
sending 2
received: 2
sending 4
received: 4
sending 6
received: 6
sending 8
received: 8

Actual result:
--------------
sending 0
received: 0
received: 
sending 4
received: 4
received: 
sending 8
received: 8