update schema 128 to properly set ttrss_feeds.last_updated default value to NULL

schema/versions/mysql/128.sql

 BEGIN;
 
+update ttrss_feeds set last_updated = NULL;
+alter table ttrss_feeds modify column last_updated datetime DEFAULT NULL;
+
 alter table ttrss_feeds add column feed_language varchar(100);
 update ttrss_feeds set feed_language = '';
 alter table ttrss_feeds change feed_language feed_language varchar(100) not null;