fix(search2): Fix NPE in PostFilterTree

[andersju]

I noticed that some search2 requests would NPE:

java.lang.NullPointerException: Cannot invoke "whelk.search2.querytree.Node.allDescendants()" because "node" is null
  at whelk.search2.querytree.EsQueryTree.flattenedConditions(EsQueryTree.java:231) ~[rest.jar:?]
  at whelk.search2.querytree.EsQueryTree$PostFilterTree.from(EsQueryTree.java:208) ~[rest.jar:?]
  at whelk.search2.querytree.EsQueryTree.<init>(EsQueryTree.java:36) ~[rest.jar:?]
  at whelk.search2.Query.doGetEsQuery(Query.java:148) ~[rest.jar:?]
  at whelk.search2.Query.getEsQuery(Query.java:344) ~[rest.jar:?]
  at whelk.search2.Query.getQueryResult(Query.java:176) ~[rest.jar:?]
  at whelk.search2.Query.getPartialCollectionView(Query.java:219) ~[rest.jar:?]
  at whelk.search2.Query.collectResults(Query.java:119) ~[rest.jar:?]
  at whelk.rest.api.SearchUtils2.doSearch(SearchUtils2.java:41) ~[rest.jar:?]
  at whelk.rest.api.SiteSearch.findData(SiteSearch.java:146) ~[rest.jar:?]
  at whelk.rest.api.Crud.handleQuery(Crud.java:155) ~[rest.jar:?]
  at whelk.rest.api.Crud.doGet2(Crud.java:133) ~[rest.jar:?]
  at whelk.rest.api.Crud.doGet(Crud.java:113) ~[rest.jar:?]

e.g. https://libris.kb.se/find.jsonld?_q=contributor%3A%22libris%3Ahftwxkx10q8ndd8%23it%22+contributor%3A%22libris%3A1zcff5tk4xvtrpw%23it%22+yearPublished%3A1937+bibliography%3A%22libris%3Alibrary%2FNB%22+bibliography%3A%22libris%3Alibrary%2FSHB%22+bibliography%3A%22libris%3Alibrary%2FNLT%22+language%3A%22lang%3Aswe%22+(library%3A%22libris%3Alibrary%2Forg%2FCTHB%22+OR+library%3A%22libris%3Alibrary%2Forg%2FBTH%22+OR+library%3A%22libris%3Alibrary%2Forg%2F3P%22+OR+library%3A%22libris%3Alibrary%2Forg%2F2K%22+OR+library%3A%22libris%3Alibrary%2Forg%2FARKM%22)&_r=itemHeldByOrg%3AARKM&_stats=true&_limit=0

It took a moment to debug because it was dependent on what ended up being activeSite, i.e., what getAndIndexDescription would load.

Adding &_site=https://id.kb.se/ makes it use the minimal configuration and then it works:

https://libris.kb.se/find.jsonld?_q=contributor%3A%22libris%3Ahftwxkx10q8ndd8%23it%22+contributor%3A%22libris%3A1zcff5tk4xvtrpw%23it%22+yearPublished%3A1937+bibliography%3A%22libris%3Alibrary%2FNB%22+bibliography%3A%22libris%3Alibrary%2FSHB%22+bibliography%3A%22libris%3Alibrary%2FNLT%22+language%3A%22lang%3Aswe%22+(library%3A%22libris%3Alibrary%2Forg%2FCTHB%22+OR+library%3A%22libris%3Alibrary%2Forg%2FBTH%22+OR+library%3A%22libris%3Alibrary%2Forg%2F3P%22+OR+library%3A%22libris%3Alibrary%2Forg%2F2K%22+OR+library%3A%22libris%3Alibrary%2Forg%2FARKM%22)&_r=itemHeldByOrg%3AARKM&_stats=true&_limit=0&_site=https://id.kb.se/

The error appears to happen when there's a multi-select library OR group and an _r filter and having _r equal to one of the values in the OR group (in this case ARKM). getFullQueryTree collapses the equal nodes but in PostFilterTree it's assumed that both are still around.

[kwahlin]

Tricky! Good job 👏