Fix data processing when trend data fetched from DB, closes #1445

pkg/datasource/zabbix.go

@@ -118,11 +118,14 @@ func (ds *ZabbixDatasourceInstance) applyDataProcessing(ctx context.Context, que
 	// Align time series data if possible
 	disableDataAlignment := query.Options.DisableDataAlignment || ds.Settings.DisableDataAlignment || query.QueryType == MODE_ITSERVICE
 	if !disableDataAlignment {
-		if useTrend && !DBPostProcessing {
+		if useTrend {
+			// Skip if data fetched directly from DB (it already contains nulls)
+			if !DBPostProcessing {
 				for _, s := range series {
 					// Trend data is already aligned (by 1 hour interval), but null values should be added
 					s.TS = s.TS.FillTrendWithNulls()
 				}
+			}
 		} else {
 			for _, s := range series {
 				// Skip unnecessary data alignment if item interval less than query interval