hook_block: explain that no check on empty block content is needed

A lot of modules I see -- mine included -- check to see if the block content they generate is empty in hook_block before setting $block['content'] and $block['subject'].
As far as I NOW know, this isn't necessary -- you can just set content and subject and Drupal core will take care of not showing anything if the content is empty.

This detail could do to be added to the docs for this hook.