Bug 28776: Warns from GetItemsInfo when biblio marked as serial

Bug 28776: Warns from GetItemsInfo when biblio marked as serial

GetItemsInfo sorts the items using serial fields if the biblio is
defined as a serial

The fields may not be defined. We can add a fallback to blank in this case

To test:
1 - Mark a record with items as serial in the marc (942$s)
2 - At least one of the items should have no enumchron defined
3 - Load the detail page
4 - Warns in logs:
    [WARN] Use of uninitialized value in string comparison (cmp) at /kohadevbox/koha/C4/Items.pm line 811.
5 - Apply patch and restart all
6 - Reload the details page
7 - No more warns

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>