Transformations: Fix Timeseries to table transformation trend reduction when result is 0 (#78026)

* Transformations: Fix Timeseries to table transformation trend reduction when result is 0

When reduceField function returns 0, the reduced value is set to null. I
think we can remove this "|| null" statement, as:
- There is a check a few lines above to make sure the field type is a number. This might be a reasonable guarantee that the calculation we are doing will make sense and there will be no need to set to null.
- Sparkline cell type already has an option to hide value if we believe the calculation
  does not make sense.

Fixes #78025

* make sure we check for undefined

---------

Co-authored-by: Oscar Kilhed <oscar.kilhed@grafana.com>

public/app/features/transformers/timeSeriesTable/timeSeriesTableTransformer.test.ts

@@ -78,6 +78,8 @@ describe('timeSeriesTableTransformer', () => {
     const series = [
       getTimeSeries('A', { instance: 'A', pod: 'B' }, [4, 2, 3]),
       getTimeSeries('B', { instance: 'A', pod: 'C' }, [3, 4, 5]),
+      getTimeSeries('C', { instance: 'B', pod: 'X' }, [4, 2, 0]),
+      getTimeSeries('D', { instance: 'B', pod: 'Y' }, [0, 0, 0]),
     ];
 
     const results = timeSeriesToTableTransform(
@@ -85,12 +87,35 @@ describe('timeSeriesTableTransformer', () => {
         B: {
           stat: ReducerID.mean,
         },
+        D: {
+          stat: ReducerID.mean,
+        },
       },
       series
     );
 
     expect(results[0].fields[2].values[0].value).toEqual(3);
     expect(results[1].fields[2].values[0].value).toEqual(4);
+    expect(results[2].fields[2].values[0].value).toEqual(0);
+    expect(results[3].fields[2].values[0].value).toEqual(0);
+  });
+
+  it('calculate the value for an empty series to null', () => {
+    const series = [getTimeSeries('D', { instance: 'B', pod: 'Y' }, [])];
+
+    const results = timeSeriesToTableTransform(
+      {
+        B: {
+          stat: ReducerID.mean,
+        },
+        D: {
+          stat: ReducerID.mean,
+        },
+      },
+      series
+    );
+
+    expect(results[0].fields[2].values[0].value).toEqual(null);
   });
 });
 

public/app/features/transformers/timeSeriesTable/timeSeriesTableTransformer.ts

@@ -172,7 +172,7 @@ export function timeSeriesToTableTransform(options: TimeSeriesTableTransformerOp
         // and push the frame with reduction
         // into the the appropriate field
         const reducerId = options[refId]?.stat ?? ReducerID.lastNotNull;
-        const value = reduceField({ field, reducers: [reducerId] })[reducerId] || null;
+        const value = reduceField({ field, reducers: [reducerId] })[reducerId] ?? null;
 
         // Push the appropriate time and value frame
         // to the trend frame for the sparkline