Set dashboard resource's widget attribute to Optional

[skarimo]

The resource submit an empty list by default so this pr sets the widget attribute to optional: https://github.com/DataDog/terraform-provider-datadog/blob/master/datadog/resource_datadog_dashboard.go#L867

This is to avoid having to provide an empty widget block when constructing a dashboard without widget. E.g.

resource "datadog_dashboard" "tfer--dashboard_uxi-002D-bcq-002D-4p7" {
  is_read_only = "false"
  layout_type  = "ordered"
  reflow_type  = "fixed"
  title        = "Test dashboard"
  widget {}
}

[skarimo]

/azp run

[azure-pipelines]

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