Fix 500 error in `GET /api/v2_alpha/notifications` when there are no notifications to return (#31746)

app/controllers/api/v2_alpha/notifications_controller.rb

@@ -77,6 +77,8 @@ class Api::V2Alpha::NotificationsController < Api::BaseController
   end
 
   def load_grouped_notifications
+    return [] if @notifications.empty?
+
     MastodonOTELTracer.in_span('Api::V2Alpha::NotificationsController#load_grouped_notifications') do
       NotificationGroup.from_notifications(@notifications, pagination_range: (@notifications.last.id)..(@notifications.first.id), grouped_types: params[:grouped_types])
     end

spec/requests/api/v2_alpha/notifications_spec.rb

@@ -116,6 +116,19 @@ RSpec.describe 'Notifications' do
 
     it_behaves_like 'forbidden for wrong scope', 'write write:notifications'
 
+    context 'when there are no notifications' do
+      before do
+        user.account.notifications.destroy_all
+      end
+
+      it 'returns 0 notifications' do
+        subject
+
+        expect(response).to have_http_status(200)
+        expect(body_as_json[:notification_groups]).to eq []
+      end
+    end
+
     context 'with no options' do
       it 'returns expected notification types', :aggregate_failures do
         subject