azurerm_monitor_metric_alert - fix crash when dynamic_criteria.0.ignore_data_before isn't set

[teowa]

resolves #21445

TF_ACC=1 go test -v ./internal/services/monitor -parallel 20 -test.run=TestAccMonitorMetricAlert_dyna -timeout 1440m -ldflags="-X=github.com/hashicorp/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccMonitorMetricAlert_dynamicCriteria
=== PAUSE TestAccMonitorMetricAlert_dynamicCriteria
=== CONT  TestAccMonitorMetricAlert_dynamicCriteria
--- PASS: TestAccMonitorMetricAlert_dynamicCriteria (389.53s)
PASS
ok      github.com/hashicorp/terraform-provider-azurerm/internal/services/monitor       389.579s

[mbfrahry]

LGTM!

[github-actions]

This functionality has been released in v3.53.0 of the Terraform Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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