Fix warning in hidden proposals spec

We were checking the proposal video URL, but its value was `nil` since
commit bedcb5bca2. This resulted in a warning:

```
Checking for expected text of nil is confusing and/or pointless since it
will always match. Please specify a string or regexp instead.
spec/system/admin/hidden_proposals_spec.rb:14
```

spec/factories/proposals.rb

@@ -80,6 +80,10 @@ FactoryBot.define do
       after(:create) { |proposal| create(:image, imageable: proposal) }
     end
 
+    trait :with_video do
+      video_url { "https://youtu.be/nhuNb0XtRhQ" }
+    end
+
     transient do
       voters { [] }
       followers { [] }

spec/system/admin/hidden_proposals_spec.rb

@@ -2,7 +2,7 @@ require "rails_helper"
 
 describe "Admin hidden proposals", :admin do
   scenario "List shows all relevant info" do
-    proposal = create(:proposal, :hidden)
+    proposal = create(:proposal, :with_video, :hidden)
     visit admin_hidden_proposals_path
 
     expect(page).to have_content(proposal.title)