Fixed ALF-10061: NodeService.getAssoc() throws an exception if the requested association does not exist

- CMIS expects to get null (as per NodeService javadoc) but it was generating a concurrency exception
 - Added a specific call to getNodeAssocOrNull to the DAO


git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@30162 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261

source/java/org/alfresco/repo/domain/node/NodeDAO.java

@@ -328,6 +328,12 @@ public interface NodeDAO extends NodeBulkLoader
      */
     public Collection<Pair<Long, AssociationRef>> getTargetNodeAssocs(Long sourceNodeId, QName typeQName);
 
+    /**
+     * @return                  Returns a specific node association with the given ID
+     *                          or <tt>null</tt> if it doesn't exist
+     */
+    public Pair<Long, AssociationRef> getNodeAssocOrNull(Long assocId);
+
     /**
      * @return                  Returns a specific node association with the given ID
      *