[web] Add key to EmptyItem

Summary: Switching to background tab that did not have any threads but EmptyItem only resulted in warning about missing key prop in list

Test Plan: Check if warning is not displayed anymore

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, zrebcu411, palys-swm, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D972