[SPA++] When circular references between documents, NPE occurs

As of the fix for HSTTWO-4273, now it shows NPE if there's a circular references between hippo documents.

Here's a reproduction steps:

• Build and run the testsuite
• Visit http://localhost:8080/site/wikipedia and create 5 wiki documents.
• Visit one wiki document and add a related document to another wiki document.
• Open the other wiki document and set the related document to the first wiki document.
• Visit resource api URL. e.g, http://localhost:8080/site/resourceapi/wikipedia/wiki-1521140740370/wiki-1521140740372/americansamoa.html
• Now you got NPE:

   Caused by: java.lang.NullPointerException
   	at com.fasterxml.jackson.databind.ser.impl.WritableObjectId.writeAsId(WritableObjectId.java:39)
   	at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase._serializeWithObjectId(BeanSerializerBase.java:596)
   	at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:148)
   	at org.hippoecm.hst.core.pagemodel.container.HippoBeanSerializer.serialize(HippoBeanSerializer.java:83)
   	at org.hippoecm.hst.core.pagemodel.container.HippoBeanSerializer.serialize(HippoBeanSerializer.java:31)
   	at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:119)
   	... 91 more
  

  15.03.2018 15:37:47 WARN  http-nio-8080-exec-1 [HstDelegateeFilterBean.doFilter:426] ContainerException 
  for 'Request{ method='GET', scheme='http', host='localhost:8080', requestURI='/site/resourceapi/wikipedia/wiki-1521142573506/wiki-1521142573508/americansamoa.html', 
  queryString='null'}': org.hippoecm.hst.core.container.ContainerException: java.lang.IllegalArgumentException: 
  (was java.lang.NullPointerException) (through reference chain: java.util.LinkedHashMap["u3ac603e964924bcb9db03a54fe25597f"]
  ->org.hippoecm.hst.core.pagemodel.container.HippoBeanWrapperModel["bean"]->org.hippoecm.hst.demo.beans.WikiBean["relatedDocs"]->java.util.ArrayList[0]
  ->org.hippoecm.hst.demo.beans.WikiBean["relatedDocs"]-
  >java.util.ArrayList[0]->org.hippoecm.hst.demo.beans.WikiBean["blocks"]->java.util.ArrayList[0])