Time series panel: Fix an issue with stacks being not complete due to the incorrect data frame length (#51910) (#51914)

* Test failure

* Fix

(cherry picked from commit 5a65a12278)

Co-authored-by: Dominik Prokop <dominik.prokop@grafana.com>

public/app/plugins/panel/timeseries/utils.test.ts

@@ -100,6 +100,8 @@ describe('prepare timeseries graph', () => {
         3,
       ]
     `);
+
+    expect(frames![0].length).toEqual(6);
   });
 
   it('will insert and convert nulls to a configure "no value" value', () => {
@@ -122,5 +124,6 @@ describe('prepare timeseries graph', () => {
         3,
       ]
     `);
+    expect(frames![0].length).toEqual(6);
   });
 });

public/app/plugins/panel/timeseries/utils.ts

@@ -104,6 +104,7 @@ export function prepareGraphableFields(
     if (hasTimeField && hasValueField) {
       frames.push({
         ...frame,
+        length: nulledFrame.length,
         fields,
       });
     }