Bug 18801 - Merging authorities has an invalid 'Default' type in the merge framework...

Bug 18801 - Merging authorities has an invalid 'Default' type in the merge framework selector

To test:
1 - Find two authorities and start a merge
2 - Leave the dropdown at 'Default'
3 - Merge records and note you get an error and can no longer view the
new record
4 - Check DB value of record authtypecode = 'Default'
5 - Apply patch
6 - Find two other authorities
7 - Merge leaving selector at default
8 - Success
9 - Check DB value of record authtypecode = ''

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