Make RegionBackendService health checks optional

[modular-magician]

Part of hashicorp/terraform-provider-google#7311

Health checks for backend services are not required when using a serverless NEG. Trying to specify a health check results in the API error:

Error: Error creating RegionBackendService: googleapi: Error 400: Invalid value for field 'resource.healthChecks': ''. 
A backend service cannot have a healthcheck with Serverless network endpoint group backends., invalid

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• 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).
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Release Note Template for Downstream PRs (will be copied)

compute: Fixed an issue where `google_compute_region_backend_service` required `healthChecks` for a serverless network endpoint group.

Derived from GoogleCloudPlatform/magic-modules#4053