There is no single "empty the trash" button in the current version of Unibox. Empty the Trash by performing the following steps:
Empty the Trash
Firstly, switch to the Trash group by using the group drop-down at the very top left of Unibox.
Secondly, select all contacts from the Contact menu
Thirdly, delete all messages by using the Contact menu again.
The above steps works pretty much the same for Junk messages. (Just select the Junk instead of the Trash group)