allow updating (R)IGM target pools to empty

[danawillow]

Fixes hashicorp/terraform-provider-google#6453.

Release Note Template for Downstream PRs (will be copied)

compute: fixed issue where removing all target pools from `google_compute_instance_group_manager` or `google_compute_region_instance_group_manager` had no effect

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 4 files changed, 191 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 4 files changed, 191 insertions(+), 1 deletion(-))

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 4 files changed, 190 insertions(+))
Terraform Beta: Diff ( 4 files changed, 190 insertions(+))