Fix display of CLI broadcast messages with user target roles

What does this MR do and why?

Broadcast messages with "Target roles" were not displayed on the CLI.

Why?

The user_access_level was always passed in as nil into the finder. So any broadcast message with a non-empty target_access_levels checked if one of its allowed levels was nil, which was always false.

See https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/system/broadcast_message.rb#L141

References

Related to #497152 (closed)

Screenshots or screen recordings

CLI change only. See instructions below.

How to set up and validate locally

• Create a new message at http://gdk.test:3000/admin/broadcast_messages
  • Make it of type banner.
  • Keep the displays on CLI checkbox enabled.
  • Leave the target roles empty for now.
• git clone any project from your GDK.
• git push a change (or git pull after creating a commit in the browser).
• This should display your messages, both on `master and this branch.
• Now edit the same broadcast message and set its target roles to include your role in the project.
• Now, on master doing another git push won't show the message 🐛

You can also verify, that if there is a newer (created_at) message, with or without target roles, that one shows instead. See https://docs.gitlab.com/ee/administration/broadcast_messages.html#banners

In the command line, only the latest banner is shown.