Fix nil reference to ShieldedNodes

[chrissng]

This PR fixes golang segmentation violation when referencing a nil ShieldedNodes object. Please see this issue for more info: hashicorp/terraform-provider-google#4756

Release Note Template for Downstream PRs (will be copied)

Fix nil reference to ShieldedNodes.

[modular-magician]

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[pdecat]

LGTM!

[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, 20bb70b.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in terraform-provider-google.
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#1314