Add an XHPAST lint rule for empty block statements

Summary: Adds an XHPAST linter rule for empty block statements. Basically, if a block statement is empty then it is much neater if the opening (`{`) and closing (`}`) braces are adjacent. Maybe this is just my own personal preference, in which case we could reduce the default severity to `ArcanistLintSeverity::SEVERITY_DISABLED`.

Test Plan: Wrote unit tests. I had to modify a bunch of existing unit tests accordingly.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10434