What is better when return false

Hope you decided to return NULL.

Returning an empty array when there are no results is not the same as returning an array with known keys. You'll have to validate the array before you can actually use it.

PHP Code:

if(NULL !== ($attachments = $model->getAttachments())) : 
# Hey, I have valid attachments and not just some returned value that looks like I do.
endif;