It should be simple, you take a look in bulk folder, you don't notice anything which may not be spam and push the "Empty" button.
But what happens if you go to toilet and push the button only after you return? Some emails may have arrived meanwhile and you will lose them. Actually you could lose bulk folder emails in the miliseconds between taking a look at the folder and decide to Empty. Some non-spam email may have arrived into it exactly at this time. Solutions? The only that comes to my mind is to delete emails manually from bulk, without using Empty button.
We know that is common for the bulk folder to contain often non-spam emails. This is because spammers become creative and they always try to create new kind of headers and content for the email to avoid anti-spam triggers.