heatmap: fix error when series empty

public/app/plugins/panel/heatmap/heatmap_ctrl.ts

@@ -146,7 +146,7 @@ export class HeatmapCtrl extends MetricsPanelCtrl {
   }
 
   onRender() {
-    if (!this.range) {
+    if (!this.range || !this.series) {
       return;
     }