Added nil guard to blue green settings in google_container_node_pool

[modular-magician]

Fixes hashicorp/terraform-provider-google#14669

Repro in hashicorp/terraform-provider-google#14669 (comment), let me know if you'd prefer we test it explicitly w/ an integration test- it's a bit annoying of a configuration to test since we can't just sub in values.

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Release Note Template for Downstream PRs (will be copied)

container: fixed a crash when all zero values are specified `google_container_node_pool.upgrade_settings.blue_green_settings.standard_rollout_policy` subfields

Derived from GoogleCloudPlatform/magic-modules#7996