Allow Update Channels to have 0 target Device Groups

[davidebriani]

When creating or updating Update Channels, at least one target Device Group must be specified currently.

However, it may happen that a user wants to leave an update channel without target groups.

For example, consider a case where the user would like to "unlink" a target group from an update channel and link it to another update channel. If the first update channel has only 1 target group, the user will not be able to accomplish the operation.