FIX: Error condition in SidebarSiteSettingsBackfiller (#19787)

app/services/sidebar_site_settings_backfiller.rb

@@ -84,6 +84,8 @@ class SidebarSiteSettingsBackfiller
       SQL
     end
 
+    return 0 if select_statements.blank?
+
     DB.query_single(<<~SQL)[0]
     SELECT
       COUNT(*)

spec/services/sidebar_site_settings_backfiller_spec.rb

@@ -226,6 +226,16 @@ RSpec.describe SidebarSiteSettingsBackfiller do
 
         expect(backfiller.number_of_users_to_backfill).to eq(3)
       end
+
+      it "returns 0 for the user count when no new category is added or removed" do
+        backfiller = described_class.new(
+          "default_sidebar_categories",
+          previous_value: "",
+          new_value: ""
+        )
+
+        expect(backfiller.number_of_users_to_backfill).to eq(0)
+      end
     end
 
     context 'for default_sidebar_tags setting' do