Bug 19673: Allow to set patron attributes to 0 with batch patron modification

Bug 19673: Allow to set patron attributes to 0 with batch patron modification

This patch will have to be tested deeply to make sure it will not
introduce regression!

The idea is to display an empty option in the patron attributes select
and ignore it. That way we can deal with false values 0 and "" which
were skipped before.

Test plan:
Add several patron attributes
Use the batch patron modification tool to add/update/remove them
Play with empty "" and 0 values, as well as other values
Modify several attributes in a row

Signed-off-by: Charles Farmer <charles.farmer@inLibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>