Bug 24759: CleanUp OpacRenewalBranch values

Bug 24759: CleanUp OpacRenewalBranch values

We had a unique behvaiour where the syspref was set to string 'NULL'
as opposed to undef, we need to clean that up

To test:
1 - Set OpacRenewalBranch to 'NULL' in staff interface
2 - Renew via opac
3 - Check statistics to ensure branch is blank

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>