[v9.5.x] Prometheus: Heatmap Format with No Data (#69096)

Prometheus: Heatmap Format with No Data (#68938)

* Prometheus: Heatmap Format with No Data

* Simplify conditional logic

(cherry picked from commit 0fa991a4b9)

Co-authored-by: Drew Slobodnjak <60050885+drew08t@users.noreply.github.com>

public/app/plugins/datasource/prometheus/result_transformer.ts

@@ -604,7 +604,7 @@ export function getOriginalMetricName(labelData: { [key: string]: string }) {
 }
 
 function mergeHeatmapFrames(frames: DataFrame[]): DataFrame[] {
-  if (frames.length === 0) {
+  if (frames.length === 0 || (frames.length === 1 && frames[0].length === 0)) {
     return [];
   }