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();

test/database/sorted.js

@@ -331,6 +331,24 @@ describe('Sorted Set methods', function () {
 				done();
 			});
 		});
+
+		it('should return empty array if keys is falsy', function (done) {
+			db.sortedSetsCard(undefined, function (err, counts) {
+				assert.ifError(err);
+				assert.equal(arguments.length, 2);
+				assert.deepEqual(counts, []);
+				done();
+			});
+		});
+
+		it('should return empty array if keys is empty array', function (done) {
+			db.sortedSetsCard([], function (err, counts) {
+				assert.ifError(err);
+				assert.equal(arguments.length, 2);
+				assert.deepEqual(counts, []);
+				done();
+			});
+		});
 	});
 
 	describe('sortedSetRank()', function () {