Prometheus: Make result transformer more robust for empty responses

- some prometheus-compatible backends may return `null` instead of empty lists
- test for lists before iterating
- added tests for empty responses

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

@@ -16,7 +16,7 @@ export class ResultTransformer {
           options.valueWithRefId
         ),
       ];
-    } else if (options.format === 'heatmap') {
+    } else if (prometheusResult && options.format === 'heatmap') {
       let seriesList = [];
       prometheusResult.sort(sortSeriesByLabel);
       for (const metricData of prometheusResult) {
@@ -24,7 +24,7 @@ export class ResultTransformer {
       }
       seriesList = this.transformToHistogramOverTime(seriesList);
       return seriesList;
-    } else {
+    } else if (prometheusResult) {
       const seriesList = [];
       for (const metricData of prometheusResult) {
         if (response.data.data.resultType === 'matrix') {
@@ -82,7 +82,7 @@ export class ResultTransformer {
     let i, j;
     const metricLabels = {};
 
-    if (md.length === 0) {
+    if (!md || md.length === 0) {
       return table;
     }
 

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

@@ -10,6 +10,31 @@ describe('Prometheus Result Transformer', () => {
     ctx.resultTransformer = new ResultTransformer(ctx.templateSrv);
   });
 
+  describe('When nothing is returned', () => {
+    test('should return empty series', () => {
+      const response = {
+        status: 'success',
+        data: {
+          resultType: '',
+          result: null,
+        },
+      };
+      const series = ctx.resultTransformer.transform({ data: response }, {});
+      expect(series).toEqual([]);
+    });
+    test('should return empty table', () => {
+      const response = {
+        status: 'success',
+        data: {
+          resultType: '',
+          result: null,
+        },
+      };
+      const table = ctx.resultTransformer.transform({ data: response }, { format: 'table' });
+      expect(table).toMatchObject([{ type: 'table', rows: [] }]);
+    });
+  });
+
   describe('When resultFormat is table', () => {
     const response = {
       status: 'success',