Null type in JSON schema ignore when generating swagger.

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the bug has not already been reported

### Fastify version

3.29.0

### Plugin version

6.0.1

### Node.js version

16.13.1

### Operating system

macOS

### Operating system version (i.e. 20.04, 11.3, 10)

12.3.1

### Description

I'm using [Typebox](https://www.npmjs.com/package/@sinclair/typebox) to generate JSON schemas for an endpoint's response. In one endpoint some response properties may be `null`.

In Typebox we defined it as follow
```javascript
Type.Union([Type.String(), Type.Null()]
```
Which seems to generate the correct JSON schema
```json
{
"anyOf": [
{ "type": "string" },
{ "type": "null" }
]
}
```

The generated swagger file does not indicate that field can be nullable (which cases some of our tests to fail).

### Steps to Reproduce

Create a schema with a property with type
```json
{
"anyOf": [
{ "type": "string" },
{ "type": "null" }
]
}
```

Generate swagger file.

### Expected Behavior

_No response_