Description:
------------
ReflectionParameter#allowsNull() return false for parameters where the nullability is inferred by a constant default value, where the constant has a value of NULL.

https://3v4l.org/mUePZ

Test script:
---------------
<?php

const DEFAULT_VALUE = null;

function foo(\stdClass $arg = DEFAULT_VALUE)
{
    var_dump($arg);
}

var_dump((new ReflectionFunction('foo'))->getParameters()[0]->allowsNull());
foo(null);

Expected result:
----------------
bool(true)
NULL

Actual result:
--------------
bool(false)
NULL