Packer build custom managed image to shared gallery, requires managed image to be set.

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#### Overview of the Issue

I have a Windows custom managed image that I’m using to build a new golden image and publish it to a Shared Image Gallery. All the necessary Azure resources are already deployed, configured, and functioning correctly.

However, when I run packer validate, I receive an error indicating that I must provide values for the managed image name and resource group.

This suggests that Packer expects to create two artifacts:

A managed image, and

A Shared Image Gallery (SIG) image version.

Based on the documentation, though, it should be possible to publish directly to the Shared Image Gallery without creating a managed image.

#### Reproduction Steps

- Builds a custom image for Azure (starting from a Marketplace or base image).
- Sets only the custom_managed_image_name and custom_managed_image_resource_group_name (not the older managed_image_* fields).
- Publishes the result to a Shared Image Gallery (SIG).
- Passes packer validate and packer build cleanly

### Packer version

From `packer version`
v1.12.0
### Simplified Packer Template

packer {
required_plugins {
azure = {
source = "github.com/hashicorp/azure"
version = "~> 2"
}
}
}
source "azure-arm" "customimage" {
virtual_network_name = var.virtual_network_name
virtual_network_resource_group_name = var.virtual_network_resource_group_name
virtual_network_subnet_name = var.virtual_network_subnet_name
subscription_id = var.subscription_id
client_id = var.client_id
client_secret = var.client_secret
location = var.location
vm_size = var.vm_size
os_type = var.os_type

communicator = ""
winrm_username = ""
winrm_password = ""
winrm_timeout = ""
winrm_insecure = true
winrm_use_ssl = true

custom_managed_image_name = var.custom_managed_image_name
custom_managed_image_resource_group_name = var.custom_managed_image_resource_group_name

shared_image_gallery_destination {
subscription = var.subscription_id
resource_group = var.gallery_resource_group
gallery_name = var.gallery_name
image_name = var.shared_image_name
image_version = var.shared_image_version
}
spot {
eviction_policy = "Delete"
max_price = "-1"
}

}

### Operating system and Environment details
windows 2022 azure edition
### Log Fragments and crash.log files

Include appropriate log fragments. If the log is longer than a few dozen lines,
please include the URL to the [gist](https://gist.github.com/) of the log or
use the [Github detailed format](https://gist.github.com/ericclemmons/b146fe5da72ca1f706b2ef72a20ac39d) instead of posting it directly in the issue.

Set the env var `PACKER_LOG=1` for maximum log detail.