Fix GH-20194: null offset deprecation not emitted for writes

[Girgias]

Based on the patch from @nielsdos and split into different commits to explain what affects what.

[alexandre-daubois]

Not approving as it's still in draft, but looks good. Thanks for taking care of this!

[arnaud-lb]

Looks good to me!

[dstogov]

You know, each new warning opens a way to modify some involved zval through the user error handler.
In this case, user error handler may probably unset() op1 and this may lead to use-after-free on attempt to add new element.

It's better to add related test(s) and fix problem at first place.

cc @iluuu1994 @nielsdos

[arnaud-lb]

Indeed:

set_error_handler(function ($errno, $errstr) {
    var_dump($errstr);
    global $a;
    unset($a);
});

$a = new stdClass;
$b = [0, null => $a];

I usually spot those, but missed this one.

I hope we can come up with a general solution to avoid these issues (#20018 / #12805).

[Girgias]

Added a test case and fix.