Steps to reproduce:

1. Create Zabbix user group (I used Server Admins).
2. Create users 2+ users.  One user does not have any media type assigned.
3. Put users into the Server Admins group
4. Configure trigger/alert.  Have alert go to Server Admin.
5. Alerts will not send as one user (out of >1 total users in group) does not have a media type.  Causes an error.

Alert should send to users who have an assigned media type.  Certainly log that a user does not have a media type configured and will not receive alerts.  But don't do a hard stop on all alerts going to all members in the group.