AnalyticsEngineDataPoint.doubles should allow null values

## Description

Currently, the `doubles` property in `AnalyticsEngineDataPoint` is strictly typed as `number[]`. This is inconsistent with the indexes and blobs fields, which both allow null values to represent missing data in specific positions.

Since Workers Analytics Engine supports sparse arrays, the current type definition forces a friction point for TypeScript users.

## Impact

TypeScript users wishing to record sparse double fields are currently prohibited from passing arrays containing null (e.g., [0, null, 1]). To work around this, users are forced to use type casting:

```ts

// Current workaround required:
doubles: [10, null, 20] as unknown as number[]
```

## Proposed Change

Update the `doubles` property to allow `null` values within the array to align with the underlying capabilities and other property definitions.

```diff
export interface AnalyticsEngineDataPoint {
indexes?: ((ArrayBuffer | string) | null)[];
- doubles?: number[];
+ doubles?: (number | null)[];
blobs?: ((ArrayBuffer | string) | null)[];
}
```

https://github.com/cloudflare/workerd/blob/main/src/workerd/api/analytics-engine.h#L33-L49