Bug 27128: (bug 25728 follow-up) Don't prefill av's code

Bug 27128: (bug 25728 follow-up) Don't prefill av's code

Bug 25728 patch "Don't prefill av's code" does not work.
Calling empty val() retruns the attribut.
Should be val('').

Test plan :
1) Without patch
2) Edit a record
3) Edit a subfield with AV
4) Enter a new value to create it
5) You see the modal popup, edit and save
6) Enter another new value to create it
7) You see the modal popup => value is prefilled
8) Re-do test plan with patch (+ clear JS cache) => value is empty

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>