Fix a remarkup rule for [x](y) links when "y" is the empty string

Summary: See https://discourse.phabricator-community.org/t/uninitialized-string-offset-error-in-phutilremarkupdocumentlinkrule/2379. This test is incorrect (should be against $uri, not $uri[0]) and should happen before we access $uri[0]).

Test Plan:

• Entered [x](), no more error in the logs.
• Entered [x](http://example.com), got a link.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20144