fixes upgrade on a empty database

src/upgrade.js

@@ -586,6 +586,14 @@ Upgrade.upgrade = function(callback) {
 			if (schemaDate < thisSchemaDate) {
 				updatesMade = true;
 
+				db.exists('topics:tid', function(err, exists) {
+					if(err) {
+						return next(err);
+					}
+					if(!exists) {
+						winston.info('[2014/2/14] Upgraded topics to sorted set - skipped');
+						return next();
+					}
 					db.getSetMembers('topics:tid', function(err, tids) {
 						if(err) {
 							return next(err);
@@ -609,6 +617,7 @@ Upgrade.upgrade = function(callback) {
 							});
 						});
 					});
+				});
 			} else {
 				winston.info('[2014/2/14] Upgrade topics to sorted set - skipped');
 				next();
@@ -620,6 +629,15 @@ Upgrade.upgrade = function(callback) {
 			if (schemaDate < thisSchemaDate) {
 				updatesMade = true;
 
+				db.exists('users:joindate', function(err, exists) {
+					if(err) {
+						return next(err);
+					}
+					if(!exists) {
+						winston.info('[2014/2/14] Added posts to sorted set - skipped');
+						return next();
+					}
+
 					db.getSortedSetRange('users:joindate', 0, -1, function(err, uids) {
 						if(err) {
 							return next(err);
@@ -649,6 +667,7 @@ Upgrade.upgrade = function(callback) {
 							next();
 						});
 					});
+				});
 
 			} else {
 				winston.info('[2014/2/14] Added posts to sorted set - skipped');