Bug 29729: Build a tr for each subscription

Bug 29729: Build a tr for each subscription

To test:
1 To to Reports > Serials > Serials subscriptions and run the report while selecting a branch without subscriptions.
2 Notice console error
3 If you run the report for a branch WITH subscriptions it works just fine.
4 Apply patch
5 Do 1 again, no console error and show no see 'No entries to show'
6 Make sure it also works when you have a few subscriptions for a branch as well

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>