Ensure levels is an array

Changelog: fixed

app/models/group.rb

@@ -224,7 +224,9 @@ def of_ancestors_and_self
   scope :excluding_restricted_visibility_levels_for_user, ->(user) do
     return all if user.can_admin_all_resources?
-    case Gitlab::CurrentSettings.restricted_visibility_levels.sort
+    levels = Array.wrap(Gitlab::CurrentSettings.restricted_visibility_levels).sort
+
+    case levels
     when [Gitlab::VisibilityLevel::PRIVATE, Gitlab::VisibilityLevel::PUBLIC],
          [Gitlab::VisibilityLevel::PRIVATE]
       where.not(visibility_level: Gitlab::VisibilityLevel::PRIVATE)

spec/models/group_spec.rb

@@ -1194,6 +1194,7 @@
         using RSpec::Parameterized::TableSyntax
         where(:restricted_visibility_levels, :expected_groups) do
+          nil                                     | lazy { [private_group, internal_group, group] }
           []                                      | lazy { [private_group, internal_group, group] }
           [private_vis]                           | lazy { [internal_group, group] }
           [internal_vis]                          | lazy { [private_group, internal_group, group] }