BigInt serialization error on Integer and Null union type

### 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

4.3.0

### Plugin version

_No response_

### Node.js version

16.16.0

### Operating system

Linux

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

elementary OS 6.1 Jólnir (Ubuntu 20.04.3 LTS)

### Description

Using typebox as the schema generator, whenever I made a type on the schema where it is a union of Integer and Null, Fastify always return as such.

```json
{
"statusCode": 500,
"error": "Internal Server Error",
"message": "Do not know how to serialize a BigInt"
}
```

With the stacktrace of,

```
TypeError: Do not know how to serialize a BigInt
at JSON.stringify ()
at anonymous12 (eval at build (/home/xxxxxx/xxxxxx/xxxxxx/node_modules/fast-json-stringify/index.js:177:23), :76:50)
at main (eval at build (/home/xxxxxx/xxxxxx/xxxxxx/node_modules/fast-json-stringify/index.js:177:23), :6:15)
at serialize (/home/xxxxxx/xxxxxx/xxxxxx/node_modules/fastify/lib/reply.js:797:12)
at preserializeHookEnd (/home/xxxxxx/xxxxxx/xxxxxx/node_modules/fastify/lib/reply.js:478:17)
at preserializeHook (/home/xxxxxx/xxxxxx/xxxxxx/node_modules/fastify/lib/reply.js:462:5)
at Reply.send (/home/xxxxxx/xxxxxx/xxxxxx/node_modules/fastify/lib/reply.js:182:7)
at Object. (/home/xxxxxx/xxxxxx/xxxxxx/index.ts:74:30)
at preHandlerCallback (/home/xxxxxx/xxxxxx/xxxxxx/node_modules/fastify/lib/handleRequest.js:126:28)
at preValidationCallback (/home/xxxxxx/xxxxxx/xxxxxx/node_modules/fastify/lib/handleRequest.js:110:5)
```

### Steps to Reproduce

1. Create a endpoint (using Typebox v0.24.20) as such
```typescript
app.get(
"/",
{
schema: {
response: {
"200": Type.Object({
test2: Type.Union([Type.Integer(), Type.Null()]),
}),
},
},
},
async (req, res) => {
return res.status(200).send({
// @ts-expect-error No BigInt Type in Typebox
test2: 12n,
});
}
);
```

3. Call the endpoint

### Expected Behavior

The serializer should serialize as an integer if it's an BigInt, and null if it's not