[9.x] Fix for model:show failing with models that have null timestamp columns

[calebdw]

Motivation

I noticed that model:show currently fails with the following error for models with a null created_at or updated_at timestamp column:

   ErrorException

  array_flip(): Can only flip string and integer values, entry skipped

  at vendor/laravel/framework/src/Illuminate/Collections/Collection.php:406
    402▕      * @return static<TValue, TKey>
    403▕      */
    404▕     public function flip()
    405▕     {
  ➜ 406▕         return new static(array_flip($this->items));
    407▕     }
    408▕
    409▕     /**
    410▕      * Remove an item from the collection by key.

      +4 vendor frames
  5   [internal]:0
      Illuminate\Foundation\Console\ShowModelCommand::Illuminate\Foundation\Console\{closure}(Object(Doctrine\DBAL\Schema\Column))

      +17 vendor frames
  23  artisan:37
      Illuminate\Foundation\Console\Kernel::handle(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))

The fix is to simply filter out any null results returned from $model->getDates() prior to flipping the collection.