Remove nil relationships links

[groyoh]

Purpose

Given this serializer:

class PostSerializer < ActiveModel::Serializer
  has_one :author do
    link :related do
      "www.example.com/author/#{object.id}" if false # the conditional is set to false to demonstrate the issue
    end
  end
end

Expected behavior

{
  "data": {
    "type": "post",
    "id": "1",
    "relationships": {
      "author": {
        "data": { "id": 1, "type": "author" }
      }
    }
  }
}

Actual behavior

{
  "data": {
    "type": "post",
    "id": "1",
    "relationships": {
      "author": {
        "data": { "id": 1, "type": "author" },
        "links": {
          "related": null
        }
      }
    }
  }
}

Changes

The nil links are now removed. This is similar as to how resource links are handled right now.

[cgmckeever]

nice!

[bf4]

@groyoh LGTM, ok to merge

[NullVoxPopuli]

I'm good with this.