Dataproc Cluster: Preserve preemptible config fields when preeemptible group removed.

[rileykarson]

Fixes hashicorp/terraform-provider-google#2861

Release Note Template for Downstream PRs (will be copied)

dataproc: fixed diff when `google_dataproc_cluster` `preemptible_worker_config.0.num_instances` is sized to 0 and other `preemptible_worker_config` subfields are set

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 2 files changed, 24 insertions(+))
Terraform Beta: Diff ( 2 files changed, 24 insertions(+))