[OAV] need to check if service follows `x-nullable`

## Description

We found that some services will return `null` value regardless of [`x-nullable` settings](https://azure.github.io/autorest/extensions/#x-nullable). By default, `x-nullable: false` which means `null` value is not allowed. So the service teams are either not aware of `x-nullable` or their implementation doesn't follow what they defined in swagger.
Live validation should check if the service follows `x-nullable`.