Adding relations to action payloads breaks scalar arrays

### Version Information

Server Version: 2.0.7

### Environment

Tested on locally hosted container, OSS

### What is the expected behaviour?

When defining payload types for actions, it is possible to define scalar arrays as part of the payload (object types are not allowed)


I can then populate this field with an array of scalar values, and it will work as expected

```js
// Request Handler
app.post('/authenticate', async (req, res) => {

// get request input
const { username, password } = req.body.input;

// run some business logic

/*
// In case of errors:
return res.status(400).json({
message: "error happened"
})
*/

// success
return res.json({
token: "",
user_id: 1,
username: "admin",
roles: ["admin", "user"] // note the array
})

});
```
Note how the `roles` field is an array of strings

Up to now this is all expected behavior.

### Keywords

Scalar Array Action Payload Relation

### What is the current behaviour?

When a relation is added to the action payload, the scalar arrays will be converted json strings.
This happens even if the relation itself is not being queried

Note the `roles` field is now an escaped json string. This is unexpected.

This is very niche behavior, but I thought it best it be documented.

Start at the action payload and relation behavior described in the issue, reproducing it through the shown request handler with a scalar array such as roles. Compare responses before and after adding a relation, including when the relation is not queried; done means scalar arrays remain arrays rather than becoming escaped JSON strings.

Written by the indexing model from the issue text.