Checking for null when no value supplied to parameter

I have a script with the following parameters, and in my code I've been testing for
$groupid -eq $null, but when it's not supplied (groupname is),

However, even if GroupName is supplied as a parameter, GroupID is not equivalent to $null.
I've found that I can test it against a blank string (if $groupid.trim() -eq '') , but I'd rather it be null - why isn't it?

[cmdletbinding(DefaultParameterSetName='GroupID')]
param (
    [Parameter(Mandatory=$true,ParameterSetName="GroupID", Position=1)][string]$GroupID,
    [Parameter(Mandatory=$true,ParameterSetName="GroupName", Position=1)][string]$GroupName,
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sirbountyAsked:
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oBdACommented:
Because you explicitly defined GroupID as string.
The best solution if you're not sure whether the object in question is an empty string or $Null is to use [string]::IsNullOrEmpty($GroupID).
Or simply use $PsCmdlet.ParameterSetName to determine which parameter set was chosen.
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sirbountyAuthor Commented:
I like the sound of the latter...I know it will be a string.
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