Bar charts should interpolate values for data based on cumulative functions

### Describe the bug

For the days where there is no additional data, the cumulative columns display null instead of carrying over the value from the day before. A cumulative value should never be null, if the previous time period has a value in there.

### To Reproduce

group by date

### Expected behavior

cumulative value should never be null, if the previous time period has a value in there. it should just carry over the value from the last period

### Logs

_No response_

### Information about your Metabase installation

```JSON
n/a
```

### Severity

medium severe

### Additional context

_No response_

Reproduce the issue by grouping a bar chart by date and inspecting cumulative values on dates with no new data. Trace the cumulative-function handling in the charting code, then add a regression test showing that missing periods carry forward the prior cumulative value and confirm the chart no longer displays null.

Written by the indexing model from the issue text.