Bug 31609: JavaScript error on Additional contents main page

Bug 31609: JavaScript error on Additional contents main page

This patch makes a minor modification to the JavaScript
on the Additional contents page to prevent an error when
there are no tabs present on the page.

The JS needs an additional check so that it doesn't run
when there are no tabs.

To test you should have more than one language installed.

- Before the patch, you would see a JavaScript error in
  the console: "Uncaught TypeError: can't access property
  "hash", firstTab[0] is undefined."
- Apply the patch and go to Tools -> Additional
  contents.
- With the patch applied there should be no error on the
  additional contents main page.
- Try creating or editing additional contents entries to
  confirm that the interface still works correctly.

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