PHP：检查x-www-form-urlencoded参数是否为null

I am trying to parse a POST (x-www-form-urlencoded) request which includes an array like this:

Now, when I loop through that:

$arr = $_POST["fieldvalue"];

foreach($arr as $key => $value)
{
    if(empty($value) || is_null($value))
    {
        echo "Something in the array is null";
    }
}

The echo-line is never done, because $value is actually "null" and not null.

That array may contain any type of value, like strings, integers, booleans, floats, ... so I don't think I can just do if($value === "null"), which does work in this example.

What I am trying to get at is that how do I parse that POST-data like so that all the "null" values get converted to proper null values, so that I can properly check for nulls?