VM.osProfile.customData always returns null

## Describe the bug

This is essentially a repost of #5866 with a little bit of additional information.

## To Reproduce

Provision a new Azure VM running Ubuntu 20.04 LTS with a defined `cloud-init` customData.

Log into the newly provisioned VM to verify that the custom data has been run and has had an effect.

Run `az vm show --resource-group groupName --name vmName` and observe that `customData` is null.

## Expected behavior

Running `az vm show --resource-group groupName --name vmName` should include an `osProfile.customData` field containing the base64 encoded custom data used to initialize the VM.

I can confirm that the custom data is run, and that the base64-encoded data is accessible from `/var/lib/waagent/ovf-env.xml` within the VM itself.

## Environment Summary
```
Linux-5.11.0-1022-azure-x86_64-with-glibc2.29
Python 3.8.10
Shell: bash

azure-cli 2.0.81

Extensions:
azure-devops 0.17.0
```

## Additional context

This is almost certainly a service related issue, as it is also visible from the Azure Resource Explorer (https://resources.azure.com/) and when issuing raw requests to the Azure Management API using cURL.

From the Azure Resource Explorer, when viewing the JSON contents of a VM, the `osProfile.customData` key is reliably missing, even though it is included in the documentation tab:

> Specifies a base-64 encoded string of custom data. The base-64 encoded string is decoded to a binary array that is saved as a file on the Virtual Machine. The maximum length of the binary array is 65535 bytes.

For using cloud-init for your VM, see [Using cloud-init to customize a Linux VM during creation](https://docs.microsoft.com/azure/virtual-machines/virtual-machines-linux-using-cloud-init?toc=%2fazure%2fvirtual-machines%2flinux%2ftoc.json)

Likewise the `osProfile.customData` field is also missing when using cURL against the API directly:

`curl 'https://management.azure.com/subscriptions/SUBSCRIPTION_ID/resourceGroups/GROUP_ID/providers/Microsoft.Compute/virtualMachines/VM_NAME?api-version=2021-03-01'`

Apologies if this isn't the right place to report such an issue— if so I'd appreciate pointers on where it might be more appropriate.

Thanks and happy new year!