Bug 28533: Set subfields to 'undef' if whole field request in itemcallnumebr system...

Bug 28533: Set subfields to 'undef' if whole field request in itemcallnumebr system preference

This patch undefines the CNsubfields variable when the whole field is requested

Note that the substr returns '' when no characters are found, so we test if the string eq ''

Testing 'truth' doesn't work because "0" is a valid option

To test:
 1 - Edit a record, add an 082 field
     082 0 0 ‡aalpha‡bbeta‡0delta
 2 - Set itemcallnumber system preference to 082
 3 - Attempt to add/edit items
 4 - ISE
 5 - Apply patch, restart all the things
 6 - Add/edit items - no error
 7 - Confirm the itemcallnumber field is populated with "alpha beta delta"
 8 - Set itemcallnumber system preference to '0820'
 9 - Add/edit items
10 - Confirm itemcallnumber is populated with 'delta'

Signed-off-by: Salman Ali <salman.ali@inLibro.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>