"Test settings" button in Manage backpacks should be displayed only when backpack has credentials

In MDL-65959 an extra option was added to backpack setting to let admins whether to configure or not the credentials for this backpack: "Include authentication details with the backpack".

However, for now, the "Test settings" button is displayed always for OBv2.0 backpacks, regardless they have or not credentials (and when there are no credentials, the test will always fail).

The button should be reviewed and displayed only for backpack with non-empty credentials.