Compact order values

Because some models use 'default_scope',
SimpleOrderBuilder cannot properly pick up order values.

Example: when a model has a default scope 'reorder(nil)'
does not actually remove `nil`:

> Event.reorder(nil).order_values
=> [nil]

app/models/event.rb

@@ -10,6 +10,7 @@ class Event < ApplicationRecord
   include UsageStatistics
   include ShaAttribute
 
+  # TODO https://gitlab.com/gitlab-org/gitlab/-/issues/358088
   default_scope { reorder(nil) } # rubocop:disable Cop/DefaultScope
 
   ACTIONS = HashWithIndifferentAccess.new(

lib/gitlab/pagination/keyset/simple_order_builder.rb

@@ -19,7 +19,9 @@ module Gitlab
 
         def initialize(scope:)
           @scope = scope
-          @order_values = scope.order_values
+          # We need to run 'compact' because 'nil' is not removed from order_values
+          # in some cases due to the use of 'default_scope'.
+          @order_values = scope.order_values.compact
           @model_class = scope.model
           @arel_table = @model_class.arel_table
           @primary_key = @model_class.primary_key

spec/lib/gitlab/pagination/keyset/simple_order_builder_spec.rb

@@ -20,6 +20,22 @@ RSpec.describe Gitlab::Pagination::Keyset::SimpleOrderBuilder do
       expect(column_definition).to be_not_nullable
       expect(column_definition).to be_distinct
     end
+
+    context "when the order scope's model uses default_scope" do
+      let(:scope) do
+        model = Class.new(ApplicationRecord) do
+          self.table_name = 'events'
+
+          default_scope { reorder(nil) } # rubocop:disable Cop/DefaultScope
+        end
+
+        model.reorder(nil)
+      end
+
+      it 'orders by primary key' do
+        expect(sql_with_order).to end_with('ORDER BY "events"."id" DESC')
+      end
+    end
   end
 
   context 'when primary key order present' do