Index model / Add new nilReason property

[fxprunayre]

Added by https://github.com/geonetwork/core-geonetwork/pull/6869#pullrequestreview-1602028121

Error was:

29/8/2023 19:28:02com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: 
Unrecognized field "nilReason" (class org.fao.geonet.index.model.gn.Link), not marked as ignorable
 (8 known properties: "protocol", "applicationProfile", "nameObject", "descriptionObject", 
"mimeType", "group", "function", "urlObject"])
29/8/2023 19:28:02 at [Source: (StringReader); line: 1, column: 34563]
 (through reference chain: org.fao.geonet.index.model.gn.IndexRecord["link"]->
java.util.ArrayList[0]->org.fao.geonet.index.model.gn.Link["nilReason"])

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