added an if to check for null to sort null as 0 (#10961)

* added an if to check for null to sort null as 0

* changed 0 to -Infinity

public/app/plugins/panel/graph/legend.ts

@@ -151,7 +151,11 @@ module.directive('graphLegend', function(popoverSrv, $timeout) {
 
         if (panel.legend.sort) {
           seriesList = _.sortBy(seriesList, function(series) {
-            return series.stats[panel.legend.sort];
+            let sort = series.stats[panel.legend.sort];
+            if (sort === null) {
+              sort = -Infinity;
+            }
+            return sort;
           });
           if (panel.legend.sortDesc) {
             seriesList = seriesList.reverse();