Don't suggest author follows for anonymous visitors (#16825)

* Don't suggest users for an anonymous visitor to follow

The sidebar raises an error when rendering suggested users for a tag
when the user is signed out.

https://app.honeybadger.io/projects/66984/faults/79342962/

* Assert empty response returned for anonymous requests

app/services/users/suggest_for_sidebar.rb

@@ -13,6 +13,8 @@ module Users
     end
 
     def suggest
+      return User.none unless user
+
       suggested_user_ids = Rails.cache.fetch(generate_cache_name, expires_in: 120.hours) do
         (reputable_user_ids + random_user_ids).uniq
       end

spec/services/users/suggest_for_sidebar_spec.rb

@@ -15,4 +15,9 @@ RSpec.describe Users::SuggestForSidebar, type: :service do
     tags = create_list(:tag, 3)
     expect(described_class.new(user, tags).suggest).to be_empty
   end
+
+  it "returns no user if not signed in" do
+    tags = "html"
+    expect(described_class.new(nil, tags).suggest).to be_empty
+  end
 end