Bug 25615: Remove empty entries in the filter dropdowns of "holds to pull"

Bug 25615: Remove empty entries in the filter dropdowns of "holds to pull"

On the "Holds to pull" view there are filters that are built on the
value of the cells.
Bug 22536 improved the ergonomic by adding a ul list, but the filters
are now broken, the ul and li tags are retrieved and put in the filter
list.

The idea of this patch is to remove the HTML tags and skip if the value
is then empty.

Test plan:
Place a hold on an item
Go to Home › Circulation › Holds to pull
Open the "libraries" filter
=> Without this patch you see empty entries
=> With this patch the filter contain only the libraries

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>