Single nil object should be treated as nil

[sr189]

In response to #350

Jbuilder is currently returning an empty array when using a partial with a single object association such as:

json.organisation @order.organisation, partial: 'organisations/organisation', as: :organisation

This behaviour was introduced by #148 in order to return an empty array in case a collection is return nil. However this behaviour leads to the fact that all nil values are rendered as an empty array even if the association does not represent a collection or the more precise collection argument is used.

[rails-bot]

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