Bug 11994: (follow-up) Make things work when LibraryName is empty

Bug 11994: (follow-up) Make things work when LibraryName is empty

When LibraryName system preference was not set, the installation
of the plugin would fail with:

Firefox could not download the search plugin from: ...

Now if the pref is empty, the description 'Library catalog'
will be used instead.

To test:
- Empty your LibraryName pref
- Verify the search plugin can still be installed
  (see first patch for further instructions)

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>