Api changes produce error in certain methods

[alglez]

Apparently there were a few changes in the ttrss API. I haven't checked in deep.

For example the subscribe method use None for login and password when there is none. But now the API expect an empty string. So there is an error raised. UTC] PHP Fatal error: Uncaught TypeError: Feeds::_subscribe(): Argument #3 ($auth_login) must be of type string, null given, called in /var/www/html/tt-rss/classes/api.php on line 830 and defined in /var/www/html/tt-rss/classes/feeds.php:1052

The problem for that case is easy to fix as def subscribe(self, feed_url, category_id=0, login="", password=""):

But probably there are other methods affected as well. I can do PR if and check other methods affected.