Check LASTEXITCODE is set before checking its value

[MBradbury]

Description

When running the Powershell provisioner with inline commands, those inline commands may not set LASTEXITCODE. However, the provisioner will always check the value of LASTEXITCODE (https://github.com/hashicorp/packer/blob/main/provisioner/powershell/provisioner.go#L41).

provisioner "powershell" {
        elevated_user     = var.winrm_username
        elevated_password = var.winrm_password
        inline = [
            "Set-StrictMode -Version Latest",
            "$ProgressPreference = 'SilentlyContinue'",
            "Write-Host \"This is a message that does not set LASTEXITCODE\""
        ]
    }

Will produce output such as the following:

2025-11-26T10:20:41Z: ==> qemu.vm: The variable '$LASTEXITCODE' cannot be retrieved because it has not been set.
2025-11-26T10:20:41Z: ==> qemu.vm: At C:\Windows\Temp\script-6926d46b-b194-43a8-7e18-73aa50bc1bdd.ps1:22 char:6
2025-11-26T10:20:41Z: ==> qemu.vm: +     if ($LASTEXITCODE -ne $null -and $LASTEXITCODE -ne 0) {
2025-11-26T10:20:41Z: ==> qemu.vm: +         ~~~~~~~~~~~~~
2025-11-26T10:20:41Z: ==> qemu.vm:     + CategoryInfo          : InvalidOperation: (LASTEXITCODE:String) [], RuntimeException
2025-11-26T10:20:41Z: ==> qemu.vm:     + FullyQualifiedErrorId : VariableIsUndefined
2025-11-26T10:20:41Z: ==> qemu.vm:

This PR checks if LASTEXITCODE has been set before checking its value.

Rollback Plan

If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No changes to security controls.

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[kp2099]

Hi @MBradbury Thank you for your contribution. This change makes sense to me. please fix the test cases as well.