If you want a column in the table to be nullable and on rollback or up function and on down function, you want it not nullable.

In Laravel version 5 you can write:

$table->...->nullable(false)->change();

For Laravel version 4 you need to write raw SQL queries:

// getting Laravel App Instance
   $app = app();
  // getting laravel main version
   $laravelVer = explode('.',$app::VERSION);
    switch ($laravelVer[0]) {
        case('5') :                
            Schema::table('pro_categories_langs', function(Blueprint $t) {
                $t->string('name', 100)->nullable()->default(null)->change();
            });               
            break;                
        default :                
             DB::statement('ALTER TABLE `pro_categories_langs` MODIFY `name` VARCHAR(100) NULL;');                                  

    }