title: Field doesn't have a default value || foreign keys
body: <p>I am trying to use a foreign key in my projectController, in order to get the name of the domain it belongs to. But, when I try to run this command : php artisan db:seed --class=ProjectTableSeeder. I got this error :
  'domain_id' doesn't have a default value.</p>
  
  <p>Any Idea please, where it comes from ?</p>
  
  <p>This my migration create_project_table :</p>
  
  <pre><code>public function up()
  {
      Schema::create('projects', function (Blueprint $table) {
          $table-&gt;increments('id', true);
          $table-&gt;string('title');
          $table-&gt;string('code');         
          $table-&gt;integer('domain_id')-&gt;nullable();
          $table-&gt;foreign('domain_id')-&gt;references('id')-&gt;on('domains');
  
          $table-&gt;timestamps();
      });
  }
  </code></pre>
  
  <p>Project_Controller :</p>
  
  <pre><code>public function create()
  {
      //
  
      $arrondissements = Arrondissement::pluck('arrondissement', 'id');
      return view('pros.create', compact('arrondissements'));
  }
  </code></pre>