Handle a potentially `NULL` Bots.LastIconUpdate and `COALESCE` back to `0`. This column has no default, and while v5.20 should never write it as `NULL`, any pre-v5.20 server writing to the new schema will. As a result, this change ensures backwards compatibility. Fixes: https://mattermost.atlassian.net/browse/MM-21645 Fixes: https://mattermost.atlassian.net/browse/MM-21585