Empty json array encodes to null.

[malud]

Describe the bug
The array may evaluate to null and therefore encodes respectively to null.

To Reproduce
Steps to reproduce the behavior:

1. Which Couper version? Run couper version or docker run --entrypoint=/couper avenga/couper version
master 2021-03-29 acfce46
2. Provide your configuration file *.hcl. Remove sensitive data.

server "custom-response" {
  endpoint "/**" {
    request {
      url = "https://httpbin.org/anything?a=${req.id}"
      body = json_encode({
        foo = []
        bar = ["bar"]
      })
    }
    response {
      body = json_encode(beresp.json_body.json)
    }
  }
}

3. Provide a curl call for reproduction

curl -v http://localhost:8080/

Result:

$ http localhost
HTTP/1.1 200 OK
Connection: close
Content-Encoding: gzip
Content-Length: 47
Date: Thu, 18 Mar 2021 18:02:26 GMT
Server: couper.io
Vary: Accept-Encoding

{"bar": ["bar"], "foo": null }

Expected behavior

{"bar": ["bar"], "foo": []}

[filex]

Could be related to #165