Example PowerShell Function that allows Null in an Integer Values

Set-AdapterVlan.ps1

function Set-AdapterVlan
{
    [CmdLetBinding()]
    param
    (
        [Parameter(Mandatory = $true)]
        $Adapter,

        [Parameter(Mandatory = $true)]
        [AllowNull()]
        [Nullable[System.Int32]]
        $VlanId
    )
    
    if ($null -eq $VlanId)
    {
        $Adapter | Set-VMNetworkAdapterVlan -Untagged
    }
    else
    {
        $Adapter | Set-VMNetworkAdapterVlan -VlanId $VlanId -Access
    }
}