Fix empty list for get aliases / shortUrls

The bug was caused because the listEntries functions was being passed the app.config.ShortUrlNamespaces slice instead of models.ShortUrlNamespaces. Unfortunately, the former was still being initialized in the test cases, therefore we did not catch this error in our tests.

I also discovered a minor postgres bug (related to handling of NULLs), which I'm fixing here too.

Closes #14 (closed)