If we have the stats FF disbabled return empty array instead of null

But we keep checking for elements in the returned object because
we don't want to include the stats property if the FF is disabled

lib/cartodb/backends/stats.js

@@ -11,6 +11,6 @@ StatsBackend.prototype.getStats = function(mapConfig, dbConnection, callback) {
     if (layerStatsEnabled) {
         layerStats().getStats(mapConfig, dbConnection, callback);
     } else {
-        return callback(null, null);
+        return callback(null, []);
     }
 };

lib/cartodb/controllers/map.js

@@ -368,7 +368,7 @@ function(req, res, mapconfig, layergroup, analysesResults, callback) {
                 if (err) {
                     return next(err);
                 }
-                if (layersStats) {
+                if (layersStats.length > 0) {
                     layergroup.metadata.layers.forEach(function (layer, index) {
                         layer.meta.stats = layersStats[index];
                     });