Revert "Remove second parameter incorrect nullable"

[LolGleb]

Reverts #1728

I made a mistake when accepted these changes. I checked what reflection returns and what is in the PHP source code. It turns out that $array is indeed an optional parameter and it's default value is null.

<?php

namespace B;

use ReflectionFunction;

$reflection = new ReflectionFunction('implode');
$parameters = $reflection->getParameters();

foreach ($parameters as $param) {
echo "Parameter: " . $param->getName();
echo "\nRequired: " . ($param->isOptional() ? "No" : "Yes");
echo "\n\n";
}

Output:
Parameter: separator
Required: Yes

Parameter: array
Required: No