fix(vm): regression: provider always tries to update cpu.affinity even if it is not specified

[bpg]

Contributor's Note

• I have added / updated documentation in /docs for any user-facing features or additions.
• I have added / updated templates in /example for any new or updated resources / data sources.
• I have updated acceptance tests to verify that the change works as expected.

Proof of Work

The new test fails before the fix:

            
            Error: error updating VM: received an HTTP 500 response - Reason: only root can set 'affinity' config
            
              with proxmox_virtual_environment_vm.test_vm5,
              on terraform_plugin_test.tf line 12, in resource "proxmox_virtual_environment_vm" "test_vm5":
              12: 				resource "proxmox_virtual_environment_vm" "test_vm5" {
            
FAIL
FAIL	github.com/bpg/terraform-provider-proxmox/fwprovider/tests	5.826s
FAIL

and passes after:

ok  	github.com/bpg/terraform-provider-proxmox/fwprovider/tests	6.012s

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Closes #1180

[mergify]

🤖 LGTM! beep boop