Fix migrations failing one time after dashboard cleanup (#14002)

The seeder tries to run the DefaultWidgetSeeder, but it doesn't exist anymore, so it fails. Add a skeleton so the migrations run right away instead of the next update.

database/seeders/DefaultWidgetSeeder.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Database\Seeders;
+
+use Illuminate\Database\Seeder;
+
+class DefaultWidgetSeeder extends Seeder
+{
+    /**
+     * Run the database seeds.
+     *
+     * @return void
+     */
+    public function run()
+    {
+        // dummy to prevent race condition blocking migrations
+    }
+}