Connection: fix getQuotaRoot crashes when box has no quota root

lib/Connection.js

@@ -903,8 +903,10 @@ Connection.prototype.getQuotaRoot = function(boxName, cb) {
       return cb(err);
 
     var quotas = {};
-    for (var i = 0, len = quotalist.length; i < len; ++i)
+    if (quotalist) {
-      quotas[quotalist[i].root] = quotalist[i].resources;
+      for (var i = 0, len = quotalist.length; i < len; ++i)
+        quotas[quotalist[i].root] = quotalist[i].resources;
+    }
 
     cb(err, quotas);
   });