Bug 29484: Make ListSets return noSetHierarchy when no sets defined

This simple patch makes our OAI-PMH server return the appropriate error
code when no sets are defined.

To test:
1. Apply the regression tests
2. Run:
   $ kshell
  k$ t/db_dependent/OAI/Server.t
=> FAIL: Tests fail. Boo!
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass! YAY!
5. Sign off :-D

Bonus: you can check with the tool mentioned on the bug report, that
the error is gone. No sets need to be defined.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: David Nind <david@davidnind.com>

JD amended patch
-    ) unless scalar @{$sets} > 0;
+    ) unless @$sets;

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>

Koha/OAI/Server/ListSets.pm

@@ -34,6 +34,17 @@ sub new {
     my $token = Koha::OAI::Server::ResumptionToken->new(%args);
     my $sets = GetOAISets;
     my $pos = 0;
+
+    return HTTP::OAI::Response->new(
+        requestURL => $repository->self_url(),
+        errors     => [
+            HTTP::OAI::Error->new(
+                code    => 'noSetHierarchy',
+                message => 'There are no OAI sets defined',
+            )
+        ],
+    ) unless @$sets;
+
     foreach my $set (@$sets) {
         if ($pos < $token->{cursor}) {
             $pos++;