DNS: always set rrdatas and routing_policy when reading record_set

[modular-magician]

Motivating bug: b/301469376

Currently on read, we only update rrdatas or routing_policy if there is a non-nil value to be read from the API response - if rrdatas or routing_policy is nil, we don't do anything. This is a problem specifically when out-of-band changes are made, since either field could be set to nil and we would never update the state to match (e.g. if rrdatas is switched to routing_policy outside of Terraform, we would not update rrdatas to nil within the Terraform state).

Setting rrdatas and routing_policy unconditionally on read (with flatten when appropriate) should resolve this.

dns: fixed record set configuration parsing in `google_dns_record_set`

Derived from GoogleCloudPlatform/magic-modules#9092

[github-actions]

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