Add nil check on bq destination array contents (#3774) (#2296)

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/3774.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+`container`: Fixed a crash in `google_container_cluster` when `""` was specified for `resource_usage_export_config.bigquery_destination.dataset_id`.
+```

google-beta/resource_container_cluster.go

@@ -2941,8 +2941,9 @@ func expandResourceUsageExportConfig(configured interface{}) *containerBeta.Reso
 		ForceSendFields: []string{"EnableNetworkEgressMetering"},
 	}
 	if _, ok := resourceUsageConfig["bigquery_destination"]; ok {
-		if len(resourceUsageConfig["bigquery_destination"].([]interface{})) > 0 {
-			bigqueryDestination := resourceUsageConfig["bigquery_destination"].([]interface{})[0].(map[string]interface{})
+		destinationArr := resourceUsageConfig["bigquery_destination"].([]interface{})
+		if len(destinationArr) > 0 && destinationArr[0] != nil {
+			bigqueryDestination := destinationArr[0].(map[string]interface{})
 			if _, ok := bigqueryDestination["dataset_id"]; ok {
 				result.BigqueryDestination = &containerBeta.BigQueryDestination{
 					DatasetId: bigqueryDestination["dataset_id"].(string),