R1:791de0886f02

Change default value for `bugtraq.url`

Summary: `null` is a more-intuitive default value than `''`, methinks.

Test Plan: Eyeball it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12671