Canonicalize option for /v1/jobs/parse-endpoint does not set defaults for ParameterizedJob Payload

# Nomad version

Output from `0.10.2`, bug also tested and verified on `0.11.0-beta2`.

### Operating system and Environment details

### Issue

When I read the documentation for ParseJob (https://nomadproject.io/api-docs/jobs/#parse-job) it says the following about `Canonicalize`:

> Canonicalize (bool: false) - Flag to enable setting any unset fields to their default values.

### Reproduction steps

```bash
curl -X "POST" "http://nomadserver.vpn.kaspergrubbe.com:4646/v1/jobs/parse" \
-H 'Accept: application/json' \
-H 'Content-Type: application/json; charset=utf-8' \
-d $'{
"Canonicalize": true,
"JobHCL": "job \\"example\\" { parameterized = { meta_required = [\\"SLEEP_TIME\\"] } type = \\"service\\" group \\"cache\\" {} }" | jq '.["ParameterizedJob"]'
}'
```

#### Output:

```json
{
"MetaOptional": null,
"MetaRequired": [
"SLEEP_TIME"
],
"Payload": ""
}
```

#### Expected output:

```json
{
"MetaOptional": null,
"MetaRequired": [
"SLEEP_TIME"
],
"Payload": "optional"
}
```

When I go to http://nomadserver.vpn.kaspergrubbe.com:4646/ui/jobs/example/definition I also see that Payload is optional there:

When reading the structs file, it also doesn't seem like an empty value is even valid: https://github.com/hashicorp/nomad/blob/d3e72883343ca5c5dbf23c72a78ac9174e55f940/nomad/structs/structs.go#L4491-L4524

Start at the /v1/jobs/parse endpoint and inspect the ParameterizedJob definition in nomad/structs/structs.go at the referenced lines. Reproduce the request with Canonicalize set to true and compare Payload with the documented default. Done means unset Payload fields receive their documented default while the existing parameterized job fields remain unchanged.

Written by the indexing model from the issue text.