SSDM-387: handle case if getProperties() returns null

SVN: 31732

datastore_server/source/java/ch/systemsx/cisd/openbis/dss/generic/server/MetaDataBuilder.java

@@ -173,6 +173,10 @@ public class MetaDataBuilder
 
     private void addProperties(String category, List<IEntityProperty> properties)
     {
+        if (properties == null)
+        {
+            return;
+        }
         Collections.sort(properties, PROPERTIES_COMPARATOR);
         for (IEntityProperty property : properties)
         {