Bugfix reindex! when all of targeted records are to delete and nothin…

…g to save (#452)

* Fixed bug at reindex! when all of targeted records are to delete and nothing to save.

* Updated not to use &. operator.

* Keep the original last_task for future refactoring.

lib/algoliasearch-rails.rb

@@ -461,7 +461,8 @@ def algolia_reindex!(batch_size = AlgoliaSearch::IndexSettings::DEFAULT_BATCH_SI
             end
             attributes.merge 'objectID' => algolia_object_id_of(o, options)
           end
-          last_task = AlgoliaSearch.client.save_objects(index_name, objects).last.task_id
+          save_tasks = AlgoliaSearch.client.save_objects(index_name, objects)
+          last_task = save_tasks.present? ? save_tasks.last.task_id : last_task
         end
         AlgoliaSearch.client.wait_for_task(index_name, last_task) if last_task and (synchronous || options[:synchronous])
       end