SAK-47814 Search avoid matching an ECP with a null ref sakaiproject#10864

Author: ottenhoff

search/elasticsearch/util/src/java/org/sakaiproject/search/elasticsearch/BaseElasticSearchIndexBuilder.java

@@ -1361,6 +1361,8 @@ public String getPDocuments() {
      */
     @Override
     public EntityContentProducer newEntityContentProducer(String ref) {
+        if (ref == null) return null;
+
         final Optional<EntityContentProducer> producer = matchEntityContentProducer(p -> p.matches(ref));
         if ( producer.isPresent() ) {
             getLog().debug("Matched content producer " + producer.get() + " for reference " + ref + " in index builder "