Fix 500 error accessing restricted visibility levels

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/22522 introduced a
regression that caused a 500 error if the
`application_settings.restricted_visibility_levels` column were `nil`.
Since there is no NOT NULL constraint, there's no guarantee that it is
populated with the proper serialized YAML data.

To fix this specific problem, we can account for `nil` values, but we
should fix this at the database level.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/215552

app/helpers/explore_helper.rb

@@ -52,7 +52,7 @@ module ExploreHelper
   end
 
   def public_visibility_restricted?
-    Gitlab::CurrentSettings.restricted_visibility_levels.include? Gitlab::VisibilityLevel::PUBLIC
+    Gitlab::CurrentSettings.restricted_visibility_levels&.include? Gitlab::VisibilityLevel::PUBLIC
   end
 
   private

changelogs/unreleased/sh-fix-error-500-explore.yml

+---
+title: Fix 500 error on accessing restricted levels
+merge_request: 30313
+author:
+type: fixed

spec/helpers/explore_helper_spec.rb

@@ -17,4 +17,25 @@ describe ExploreHelper do
       expect(helper.explore_nav_links).to contain_exactly(*menu_items)
     end
   end
+
+  describe '#public_visibility_restricted?' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:visibility_levels, :expected_status) do
+      nil | nil
+      [Gitlab::VisibilityLevel::PRIVATE] | false
+      [Gitlab::VisibilityLevel::PRIVATE, Gitlab::VisibilityLevel::INTERNAL] | false
+      [Gitlab::VisibilityLevel::PUBLIC] | true
+    end
+
+    with_them do
+      before do
+        stub_application_setting(restricted_visibility_levels: visibility_levels)
+      end
+
+      it 'returns the expected status' do
+        expect(helper.public_visibility_restricted?).to eq(expected_status)
+      end
+    end
+  end
 end