Bug 31269: Define sDefaultContent for DataTables

Bug 31269: Define sDefaultContent for DataTables

1. Stage a MARC record for import
2. Once you have one click on 'Manage staged records'
3. You will see an alert that says "Something went wrong when loading the table."
4. If you dismiss that you should see the following error:

DataTables warning: table id=records-table - Requested unknown parameter '' for row 0, column 4. For more information about this error, please see http://datatables.net/tn/4

5. Appy patch
6. Try ssteps 1 & 2 again
7. No DataTable errors

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>