Declaration affirmation SignatorySerializer.java not serializing for XML

SignatorySerializer.java not serializing for XML, but fine for JSON.

Working example: JSON
```
      "signatories" : [
        {
          "organization" : {
            "name" : "Acme Inc"
          },
          "externalReference" : {
            "type" : "website",
            "url" : "https://acme.inc.com"
          }
        }
      ]
```

Same example: XML
```
      <signatories/>
```
