if google_compute_region_disk_resource_policy_attachment is empty, shouldn't error

[megan07]

Fixes hashicorp/terraform-provider-google#6059

Release Note Template for Downstream PRs (will be copied)

compute: fixed error where plan would error if `google_compute_region_disk_resource_policy_attachment` had been deleted outside of terraform.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 13 files changed, 13 insertions(+), 39 deletions(-))
Terraform Beta: Diff ( 17 files changed, 17 insertions(+), 51 deletions(-))