Bug 20468: (QA follow-up) Deactivate select/clear all for empty tab

Bug 20468: (QA follow-up) Deactivate select/clear all for empty tab

This only pertains to the second (Processing) tab.

Test plan:
[1] Have one entry in Pending and zero in Processing.
[2] Click on Processing tab.
[3] Without this patch, you would see Select all/Clear all.
    With this patch, you won't.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>