Bug 17380: Do not use GuessAuthTypeCode in MetadataRecord::Authority

Bug 17380: Do not use GuessAuthTypeCode in MetadataRecord::Authority

If we got an authtypecode from the database and this value is not NULL
since the table column does not allow it, there is no need to call
GuessAuthTypeCode for empty string (read: Default framework) in the
sub get_from_authid.

Furthermore, we remove three Koha::MetadataRecord::Authority->new calls.
They are useless, since we do not pass a record. It just generates:
  No record passed at authorities/merge.pl line 96.
  Can't bless non-reference value at Koha/MetadataRecord/Authority.pm line 66.
Instead we throw an ObjectNotFound exception.

Test plan:
[1] Run t/db_dependent/Koha_Authority.t
[2] Interface will be tested in the following patches.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>