aws-events: Support 'null' values for event matching

I'm trying to use AWS CDK to create an eventbridge rule.

The custom event will have a detail payload where I only want to match on events where order_item_id is null.

Here is the rule in cdk:

```
rule.addEventPattern({
account: ['xxxxxxxxx'],
detailType: ['inventory_adjusted'],
detail: {
'order_item_id': [null]
}
})
```

And according to aws documentation, this is allowed:

https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/CloudWatchEventsandEventPatterns.html

> To match events where the value of responseElements is null, use the following pattern, which would match the event example.
> { "detail": { "responseElements": [null] } }

However, when deploying with cdk I get this error:

> development failed: ValidationError: [/Resources/sendinventoryadjustmentstolambda8954C2E5/Type/EventPattern/detail/order_item_id/0] 'null' values are not allowed in templates >[/Resources/sendinventoryadjustmentstolambda8954C2E5/Type/EventPattern/detail/order_item_id/0] 'null' values are not allowed in templates

And here is the cdk synth for the area around the rule:

```
EventPattern:
account:
- "xxxxxxxxx"
detail-type:
- inventory_adjusted
detail:
order_item_id:
- null
```

Indeed it is null... which apparently isn't allowed in yaml template, but then how do I express this in CDK?

### Reproduction Steps

create a rule that tries to match a null value for detail key

```
rule.addEventPattern({
account: ['xxxxxxxxx'],
detailType: ['inventory_adjusted'],
detail: {
'order_item_id': [null]
}
})
```

### Error Log

> development failed: ValidationError: [/Resources/sendinventoryadjustmentstolambda8954C2E5/Type/EventPattern/detail/order_item_id/0] 'null' values are not allowed in templates >[/Resources/sendinventoryadjustmentstolambda8954C2E5/Type/EventPattern/detail/order_item_id/0] 'null' values are not allowed in templates

### Environment

- **CLI Version :
```
cdk --version
1.45.0 (build 0cfab15)
```

- **Framework Version:**
- **Node.js Version:

```
node -v
v11.15.0
```

- **OS : osX 10.14.6
- **Language (Version): typescript

### Other

---

This is :bug: Bug Report

Start with the TypeScript rule.addEventPattern call and the synthesized EventPattern shown in the report. Trace how the detail.order_item_id value reaches template validation, then compare the behavior with the AWS event-pattern documentation. Done means a null matcher can be synthesized and deployed without the reported validation error.

Written by the indexing model from the issue text.