check if instance manager not found on delete, don't return an error

[modular-magician]

Fixes hashicorp/terraform-provider-google#14516

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Release Note Template for Downstream PRs (will be copied)

compute: fixed crash on `terraform destroy -refresh=false` for instance group managers with `wait_for_instances = "true"` if the instance group manager was not found

Derived from GoogleCloudPlatform/magic-modules#7903