$GetChildItemSplat = @{
    Path = "C:\Temp"
    LiteralPath = ""
    Filter = "*.txt"
    Recurse = $True
    Depth = $null
}

#This will throw an exception telling you 'An error occurred while enumerating through a collection: Collection was modified; enumeration operation may not execute'
$GetChildItemSplat.Keys |
  Where-Object -FilterScript {$GetChildItemSplat[$_] -eq $Null} |
  ForEach-Object -Process { $GetChildItemSplat.Remove($_) }

#This will work
Foreach($Key in $($GetChildItemSplat.Keys))
{
    If($Null -eq $GetChildItemSplat[$Key] -or $GetChildItemSplat[$Key] -eq "")
    {
        $GetChildItemSplat.Remove($Key)
    }
}