Use updated policy to update, not empty policy for DEPRIVILEGE

[modular-magician]

Fixes: hashicorp/terraform-provider-google#7871

We previously were setting the IAM policy to an empty policy any time DEPRIVILEGE would take action

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Release Note Template for Downstream PRs (will be copied)

project: fixed a bug where `google_project_default_service_accounts` would delete all IAM bindings on a project when run with `action = "DEPRIVILEGE"`

Derived from GoogleCloudPlatform/magic-modules#4293