prevent quoted null when to_json is called on nil relationship

[soopa]

This is an update to #23 from 2011 to target the release-1.2 branch rather than master given the divergence of those two branches, and abandonment of master. The impact and code change is the same as before…

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Fixes the case where calling something like:

resource.to_json(:relationships => { :nested_resource => { :only => [:id] } })

When the nested resource returns nil, would return invalid JSON like…

{
  "id":1,
  "nested_resouce":"null"
}

Rather than valid JSON like…

{
  "id":1,
  "nested_resouce":null
}

(Note the quotes around null)