How can I prevent a string argument changing from null to empty when bound to a parameter?

Question

Consider the following code:

function f {
    param (
        [AllowNull()]
        [string]
        $x
    )
    return $x
}

$r = f -x $null

Answer 1

function f {
    param (
        [AllowNull()]$x
    )
    return $x
}

$r = f -x $null

By removing the [string] and using [AllowNull()] the above function will now allow you to pass in a null object or an empty string. You can check for the type using $x.GetType with an if statement and determining if $x is null or an empty string.