Add default_if_empty to google_compute_router_nat defaults

[rileykarson]

More defaults I picked up during the same run as #2943

Release Note Template for Downstream PRs (will be copied)

compute: fixed `google_compute_router_nat` timeout fields causing a diff when using a long-lived resource

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 9c355b8.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#1613
depends: hashicorp/terraform-provider-google#5353

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 0f90bdf.

Pull request statuses

terraform-provider-google-beta already has an open PR.
No diff detected in terraform-google-conversion.
terraform-provider-google already has an open PR.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I didn't open any new pull requests because of this PR.

[rileykarson]

Sorry, this ended up changing a bit! There was an assumption that v was a string that I missed. Mind taking another look?

[c2thorn]

Nice generalizing the empty check