Fix inconsitency between general processing rules and pagination links (#1606)
Fix inconsitency between general processing rules and pagination links (#1606)

* Fix inconsitency between general processing rules and pagination links

`null` value was not listed as an allowed value for links in general processing rules for links objects. But it was allowed explicitly for pagination links. This change fixes that inconsitency by allowing `null` for _every_ link to indicate that it does not exist.

* Update _format/1.1/index.md
json-api/json-api,json-api/json-api,json-api/json-api,json-api/json-api,json-api/json-api