mysql: make 'user' actually optional in the DB connection string

neo/storage/database/mysqldb.py

@@ -117,7 +117,9 @@ class MySQLDatabaseManager(DatabaseManager):
         return super(MySQLDatabaseManager, self).__getattr__(attr)
 
     def _tryConnect(self):
-        kwd = {'db' : self.db, 'user' : self.user}
+        kwd = {'db' : self.db}
+        if self.user:
+            kwd['user'] = self.user
             if self.passwd is not None:
                 kwd['passwd'] = self.passwd
         if self.socket: