dont return undefined from sortedSetsCard if keys is empty or falsy

src/database/mongo/sorted.js

@@ -149,7 +149,7 @@ module.exports = function (db, module) {
 
 	module.sortedSetsCard = function (keys, callback) {
 		if (!Array.isArray(keys) || !keys.length) {
-			return callback();
+			return callback(null, []);
 		}
 		var pipeline = [
 			{ $match: { _key: { $in: keys } } },

src/database/postgres/sorted.js

@@ -231,7 +231,7 @@ SELECT COUNT(*) c
 
 	module.sortedSetsCard = function (keys, callback) {
 		if (!Array.isArray(keys) || !keys.length) {
-			return callback();
+			return callback(null, []);
 		}
 
 		query({

src/database/redis/sorted.js

@@ -116,7 +116,7 @@ module.exports = function (redisClient, module) {
 	};
 
 	module.sortedSetsCard = function (keys, callback) {
-		if (Array.isArray(keys) && !keys.length) {
+		if (!Array.isArray(keys) || !keys.length) {
 			return callback(null, []);
 		}
 		var batch = redisClient.batch();