[AdminUI] Roles should require at least one channel to be selected

[halykon]

Describe the bug
When creating a role in the Admin UI, the channel for the permissions can be empty. If no channel is selected, no permissions will be applied, therefore all @Allow() auth guards will deny access, even if the role has the correct permission

To Reproduce
Steps to reproduce the behavior:

1. Open the Admin UI
2. Click on Settings > Roles
3. Check the Channel input, it is allowed to be empty.
4. Create a role without a channel, assign it the correct permissions and to a user
5. Try to access a protected Query

Expected behavior
The channel selection input should not allow empty values and the Admin API might want to require the channel as well. That would prevent an empty channel to be set, therefore, permissions will always work as expected

Environment (please complete the following information):

• @vendure/core version: 2.1.8
• Nodejs version: v18.19.1
• Database (mysql/postgres etc): postgres