Ignore relationships that don't have data property

[pdw-mb]

I'm using json-api-merge on a request that only has some relationships included, so I have data that looks like this:

  "relationships": {
    "validation_messages": {
      "links": {
        "related": "[/api/filings/5877/validation_messages](http://localhost:5000/api/filings/5877/validation_messages)"
      }
    },
    "entity": {
      "links": {
        "related": "[/api/entities/213800XB8YZNGJYDEZ97](http://localhost:5000/api/entities/213800XB8YZNGJYDEZ97)"
      },
      "data": {
        "type": "entity",
        "id": "1331"
      }
    }
  }

Note the lack of data property on validation_messages. json-api-merge currently breaks on this input. This PR filters relationships to those which contain the data property.

[char0n]

Hi @pdw-mb,

You're right. According to the https://jsonapi.org/format/#document-resource-object-relationships, one of links, meta, data must be contained within relationships. Thus we cannot assume that data is always there.

[char0n]

Added test for asserting that json-api-merge throws error on the case you described: #788

[char0n]

I've fixed linting issue + changed the test to verify your change.

[char0n]

Thanks for contributing.

[char0n]

Released in https://www.npmjs.com/package/json-api-merge/v/1.1.4