Fix TypeError caused by empty lists being passed as a string to re.sub.

In make_api_response, empty lists were not JSONized because they were
mistakenly detected as a None, which caused these empty lists to be
passed to urlize_links_and_mails, which expects strings.

It caused a TypeError when calling re.sub() on it, which was catched
so it still returned a result (and a correct one, since empty lists
don't need substitution).

This commit removes the catch workaround and always passes the right
type.