fix: Instance state has placement groups by default

📝 Description

Fixed issue causing an empty list of placement groups gets added to the instance state, even when none are defined.

✔️ How to Test

The following steps assume that you pulled down this PR locally.

First, verify that instance-related integration tests are passing, i.e. make PKG_NAME=linode/instance ARGS="-run TestAccDataSourceInstances_basic" int-test

Generate a compliant firewall in Cloud Manager.
Initialize terraform in a sandbox environment, i.e. dx-devenv.

Create a placement group and instance resource with the firewall's id, and assign the instance to that PG


resource "linode_placement_group" "test" {
label = "my-placement-group"
region = "us-mia"
placement_group_type = "anti_affinity:local"
}

resource "linode_instance" "foobar" { label = "test-tf-instance" type = "g6-standard-1" region = "us-mia" image = "linode/alpine3.19" firewall_id = placement_group { id = linode_placement_group.test.id } }


4. Go to Cloud Manager and unassign the instance from the PG
5. Run `make plan` and verify that there is an update in-place to add the instance back to the PG
6. Destroy all resources created for this test

linode / terraform-provider-linode

fix: Instance state has placement groups by default #1619

📝 Description

✔️ How to Test