title: Laravel migration from nullable to not null with null data in database
body: <p>my code below is working fine. Problem is when i got data in database which is null in user_id field. Migration rollback will not pass successfully. What is good approach to solve this problem?
  Should i change every null in column user_id to empty string before i change nullable to false in rollback?</p>
  <pre><code>    public function up()
      {
          Schema::table('classes', function (Blueprint $table) {
              $table-&gt;text('user_id')-&gt;nullable()-&gt;change();
          });
      }
      
      /**
       * Reverse the migrations.
       *
       * @return void
       */
      public function down()
      {
          Schema::table('classes', function (Blueprint $table) {
              $table-&gt;text('user_id')-&gt;nullable(false)-&gt;change();
          });
      }
  </code></pre>