Fix ArcanistCSSLintLinter issue for messages without line number

Summary:
csslint offers some diagnostics related to all the document without any
relevant line number.

In such case, arc lint failed, as setLine expects null or an integer,
but not an empty string.

The 'char' and 'line' attributes in XML report are now optional.

Fixes T9804.

Test Plan: test case to repro the issue added

Reviewers: chad, joshuaspence, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T9804

Differential Revision: https://secure.phabricator.com/D14497