[field formatters] IP should return - on null or undefined

[jbudz]

Null ip addresses were turning into 0.0.0.0 after running through the ip formatter. This changes null/undefined to render as -.

POST /foo/bar
PUT /foo/bar/_mapping
{
  "properties": {
    "test": {
      "type": "ip"
    }
  }
}
POST /foo/bar/1
{
  "test": "192.168.1.1"
}
POST /foo/bar/2
{
  "test": null
}

Closes #7182

[LeeDr]

LGTM