Add nil check on bq destination array contents

[slevenick]

Fixes: hashicorp/terraform-provider-google#6814

Release Note Template for Downstream PRs (will be copied)

`container`: Fixed a crash in `google_container_cluster` when `""` was specified for `resource_usage_export_config.bigquery_destination.dataset_id`.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 3 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 1 file changed, 3 insertions(+), 2 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=134875"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccMonitoringMetricDescriptor_monitoringMetricDescriptorAlertExample You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=134876"