collect vm screenshot throws error after vm is off

... this error noise is confusing:

https://github.com/microsoft/openvmm/actions/runs/18592557887/job/53012618778?pr=2198

    [     petri] 2025-10-17T12:54:09.168098Z DEBUG petri: command exited in 1.127s with status exit code: 0 cmd="\"powershell.exe\" \"-NoProfile\" \"Import-Module\" \"\\\"C:\\\\Windows\\\\SystemTemp\\\\.tmpDjRmW8\\\\hyperv.psm1\\\"\" \";\" \"Get-VM\" \"-Id\" \"\\\"4cb79f6b-1841-444d-8070-8653f7ba59da\\\"\" \"|\" \"Get-VmScreenshot\" \"-Path\" \"\\\"C:\\\\Windows\\\\SystemTemp\\\\.tmpDjRmW8\\\\screenshot.bin\\\"\"" stdout_str=",\r\n" stderr_str="Invoke-CimMethod : Invalid method Parameter(s) \r\nAt C:\\Windows\\SystemTemp\\.tmpDjRmW8\\hyperv.psm1:445 char:22\r\n+ ... e = $vmms | Invoke-CimMethod -MethodName \"GetVirtualSystemThumbnailIm ...\r\n+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\r\n    + CategoryInfo          : NotSpecified: (Msvm_VirtualSys...T-CC-S1523-10\"):CimInstance) [Invoke-CimMethod], CimExc \r\n   eption\r\n    + FullyQualifiedErrorId : HRESULT 0x8004102f,Microsoft.Management.Infrastructure.CimCmdlets.InvokeCimMethodCommand\r\n \r\nException calling \"WriteAllBytes\" with \"2\" argument(s): \"Value cannot be null.\r\nParameter name: bytes\"\r\nAt C:\\Windows\\SystemTemp\\.tmpDjRmW8\\hyperv.psm1:451 char:5\r\n+     [IO.File]::WriteAllBytes($Path, $image.ImageData)\r\n+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\r\n    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException\r\n    + FullyQualifiedErrorId : ArgumentNullException\r\n \r\n"
    [     petri] 2025-10-17T12:54:09.168246Z ERROR petri::vm: Failed to take screenshot e=invalid x dimension

    Caused by:
        cannot parse integer from empty string

    Stack backtrace:
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Start with the linked GitHub Actions run and trace the screenshot-collection path around the PowerShell Get-VmScreenshot invocation when the VM is off. Confirm the expected behavior for an unavailable screenshot; done means this condition no longer produces confusing error noise or a follow-on parse failure.