CloudWatch: Fix segfault when migrating legacy queries (#93543)

pkg/tsdb/cloudwatch/models/cloudwatch_query.go

@@ -385,7 +385,11 @@ func (q *CloudWatchQuery) validateAndSetDefaults(refId string, metricsDataQuery
 func getStatistic(query metricsDataQuery) string {
 	// If there's not a statistic property in the json, we know it's the legacy format and then it has to be migrated
 	if query.Statistic == nil {
-		return query.Statistics[0]
+		if len(query.Statistics) > 0 {
+			return query.Statistics[0]
+		}
+		// if there isn't a statistic property in the legacy format fall back to Average
+		return "Average"
 	}
 	return *query.Statistic
 }

pkg/tsdb/cloudwatch/models/cloudwatch_query_test.go

@@ -296,6 +296,37 @@ func TestRequestParser(t *testing.T) {
 		assert.Equal(t, "Average", migratedQuery.Statistic)
 	})
 
+	t.Run("legacy statistics field is migrated: if no stat, uses Average", func(t *testing.T) {
+		oldQuery := []backend.DataQuery{
+			{
+				MaxDataPoints: 0,
+				QueryType:     "timeSeriesQuery",
+				Interval:      0,
+				RefID:         "A",
+				JSON: json.RawMessage(`{
+				   "region":"us-east-1",
+				   "namespace":"ec2",
+				   "metricName":"CPUUtilization",
+				   "dimensions":{
+						"InstanceId": ["test"]
+					},
+				   "statistics":[],
+				   "period":"600",
+				   "hide":false
+				}`),
+			},
+		}
+
+		migratedQueries, err := ParseMetricDataQueries(oldQuery, time.Now(), time.Now(), "us-east-2", logger, false)
+		assert.NoError(t, err)
+		require.Len(t, migratedQueries, 1)
+		require.NotNil(t, migratedQueries[0])
+
+		migratedQuery := migratedQueries[0]
+		assert.Equal(t, "A", migratedQuery.RefId)
+		assert.Equal(t, "Average", migratedQuery.Statistic)
+	})
+
 	t.Run("New dimensions structure", func(t *testing.T) {
 		query := []backend.DataQuery{
 			{