refactor(insights): share download data normalization (#4770)

## Description

JSON.stringify previously handled invalid Date values as null through
Date.toJSON before its replacer ran. Normalize
Insights columns and row values once instead so CSV and JSON share
explicit date validation while preserving their existing output: CSV
presents null as an empty field and JSON presents it as null.

```
    Insights result -> normalized export rows --> CSV presenter
                                              \-> JSON presenter
```

## Release note

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notes. -->
None.

## Migration note

<!-- Upgrade, deployment, config, schema, or compatibility notes. Use
"None" if none. -->
None.

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Co-authored-by: Albert Chae <albertchae@users.noreply.github.com>