DSpaceObject.getIdentifier(Context) should never return null.

If an DSO does not have any identifiers (including handle) this method
should return an empty string array. G.e. an item during submission
(workspace item) has no identifiers at all. This commits prevents NPE in
event handling classes.

dspace-api/src/main/java/org/dspace/content/DSpaceObject.java

@@ -134,6 +134,13 @@ public abstract class DSpaceObject
             }
         }
 
+        // it the DSO has no identifiers at all including handle, we should return an empty array.
+        // G.e. items during submission (workspace items) have no handle and no other identifier.
+        if (identifiers == null)
+        {
+            identifiers = new String[] {};
+        }
+
         if (log.isDebugEnabled())
         {
             StringBuilder dbgMsg = new StringBuilder();