[BUG]: AzurePowerShell@5: Fallback to Get-LatestModuleLinux fails when version is set to "null"

[pkosek]

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Task name

AzurePowerShell

Task version

5

Issue Description

The AzurePowerShell@5 task fails on Linux agents when azurePowerShellVersion is not specified, due to a bug in the script logic in Utility.ps1.

When no version is provided in the YAML CI, the $targetAzurePs parameter is set to the string "null", not $null, which causes the fallback logic to be skipped and leads to an invalid path like /usr/share/az_null.

This results in the following error, even when the Az module is preloaded:

Az version null not available locally on the agent. Downloading dynamically.
Validate-VersionParameters: Cannot convert value 'null' to type 'System.Version'.

Environment type (Please select at least one enviroment where you face this issue)

• Self-Hosted
• Microsoft Hosted
• VMSS Pool
• Container

Azure DevOps Server type

dev.azure.com (formerly visualstudio.com)

Azure DevOps Server Version (if applicable)

No response

Operation system

Ubuntu 22.04

Relevant log output

/usr/bin/pwsh -NoLogo -NoProfile -NonInteractive -ExecutionPolicy Unrestricted -Command . '/agent/_work/_temp/9345e305-301d-4cac-8ceb-51e68b5473a1.ps1'
File saved!
VERBOSE: Module path not present as expected in hosted agent, skipping step to make module available.
##[debug]Processed: ##vso[telemetry.publish area=TaskHub;feature=AzurePowerShellV5]{"targetAzurePs":"null","moduleSource":"privateAgent"}
VERBOSE: The value of the module path is: /usr/share/az_null
VERBOSE: No module path found with this name
VERBOSE: The updated value of the PSModulePath is: /home/AzDevOps/.local/share/powershell/Modules:/usr/local/share/powershell/Modules:/opt/microsoft/powershell/7/Modules
Exception: /agent/_work/_tasks/AzurePowerShell_72a1931b-effb-4d2e-8fd8-f8472a07cb62/5.257.0/Utility.ps1:94
Line |
  94 |  …             throw ("Could not find the module path with given version …
     |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | Could not find the module path with given version.

##[debug]Exit code 1 received from tool '/usr/bin/pwsh'
##[debug]STDIO streams have closed for tool '/usr/bin/pwsh'
##[debug]task result: Failed
##[error]PowerShell exited with code '1'.

Full task logs with system.debug enabled

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Repro steps