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

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>