Bug 32745: Adjust context for invalid background jobs

We had some problems recently and it's possible to have
background_jobs.context=NULL
But the REST API specs is expecting an object.
We could either modify the specs, or adjust the incorrect entries in DB,
which is IMO better.

Test plan:
0. Don't apply the patch
1. Create some background jobs
2. Set context to NULL
UPDATE background_jobs SET context=NULL;
3. List the jobs and note the datatable error, and the error in Koha log
  OpenAPI >>> GET api/v1/jobs [{"message":"Expected object - got null.","path":"\/body\/0\/context"},{"message":"Expected object - got null.","path":"\/body\/1\/context"}]

4. Apply the patch
5. Run `updatedatabase`
6. Confirm that the list and detail view are not working correctly

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

installer/data/mysql/atomicupdate/bug_32745.pl

@@ -0,0 +1,15 @@
+use Modern::Perl;
+
+return {
+    bug_number => "32745",
+    description => "Update context={} for invalid background jobs",
+    up => sub {
+        my ($args) = @_;
+        my ($dbh, $out) = @$args{qw(dbh out)};
+        $dbh->do(q{
+            UPDATE background_jobs
+            SET context="{}"
+            WHERE context IS NULL
+        });
+    },
+};