Fix NodeDAO.getStore to do what the JavaDoc says it does

- Fixed AbstractAuthorityBridgeDAO to detect potential null


git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@48650 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261

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

@@ -697,7 +697,7 @@ public abstract class AbstractNodeDAOImpl implements NodeDAO, BatchingDAO
         Pair<StoreRef, Node> rootNodePair = rootNodesCache.getByKey(storeRef);
         if (rootNodePair == null)
         {
-            throw new InvalidStoreRefException(storeRef);
+            return null;
         }
         else
         {