Fixes #36521 - raise an error with invalid manifest

[lfu]

What are the changes introduced in this pull request?

Considerations taken when implementing this change?

What are the testing steps for this pull request?

1. Export a Red Hat repository
2. Go to your disconnected Satellite and remove all subscriptions from your manifest (DO NOT DELETE THE MANIFEST, JUST LET IT EMPTY)
3. Try importing the Red hat repository exported in step 1

Before:

Could not import the archive.:
undefined method `substitutor' for nil:NilClass

After

Could not import the archive.:
  Manifest does not have a valid subscription

[theforeman-bot]

Issues: #36521

[chris1984]

On my export org ( with a current manifest that has subscriptions) When trying to enable a RH repo I get the following

undefined method `empty?' for #Katello::Resources::CDN::CdnResource:0x00007fa5151051c8

Looks like the 2 failing tests are also failing with the same thing

https://ci.theforeman.org/blue/organizations/jenkins/katello-pr-test/detail/katello-pr-test/12939/tests

[chris1984]

Works great,

Exported a repo with my default organization
Created a new organization with an empty manifest and tried to import

[root@centos8-stream-katello-nightly ~]# hammer content-import repository --organization-id 3 --path /var/lib/pulp/imports/Default_Organization/Export-Red_Hat_Satellite_Client_6_for_RHEL_7_Server_RPMs_x86_64-1/1.0/2023-06-26T15-40-49-00-00/
Could not import the archive.:
  The organization's manifest does not contain the subscriptions required to enable the following repositories.
   
  * Product = 'Red Hat Enterprise Linux Server', Repository = 'Red Hat Satellite Client 6 for RHEL 7 Server RPMs x86_64'