Properly handle AutomatedBackupPolicy midnight start time (hours = 0)

[modular-magician]

Currently, if you set Hours = 0 in Cluster.AutomatedBackupPolicy, the change will properly be applied, but when the resource is read, it treats 0 as an empty configuration. This leads to terraform plan returning incorrect output, stating that it will update the weekly schedule, even though it is already set.

I consider this a small bug-fix, so I did not open an issue in issue tracker.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
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Release Note Template for Downstream PRs (will be copied)

Fix automated backup policy handling of 0 start time (midnight)

Derived from GoogleCloudPlatform/magic-modules#8337