Bug 36832: (bug 36791 follow-up) Allow authid=0

To test:
1. Go to the Authorities module and do a Z39.50 search that will return
   results (e.g. a general subject heading)
2. Import an authority record from the results
--> Confirm that the record is imported into the editor
3. Authorities > New, replace it via Z39.50, confirm it's imported
4. Authorities > New, fill the required fields, confirm it saves
5. Edit an existing authority, replace it via Z39.50, confirm it saves
6. Edit an existing authority, replace authid=nn in the URL with a
   number that doesn't exist like 1000000, confirm you get a 404 page

Signed-off-by: Phil Ringnalda <phil@chetcolibrary.org>
Signed-off-by: Janusz Kaczmarek <januszop@gmail.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 1aba01adb4)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit aa7bea04c4)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>

authorities/authorities.pl

@@ -548,7 +548,7 @@ my $changed_authtype = $input->param('changed_authtype') // q{};
 
 my $dbh = C4::Context->dbh;
 my $authobj = Koha::Authorities->find($authid);
-if ( defined $authid && !$authobj ) {
+if ( $authid && !$authobj ) {
     print $input->redirect("/cgi-bin/koha/errors/404.pl");    # escape early
     exit;
 }