Fix PHP notice when running phpunit tests
Fix PHP notice when running phpunit tests

When running phpunit, this method is being called, which calls ArticleMetadata::getValidTags(), which produces a query to pagetriage_tags.
Unittests will execute this query: SELECT  ptrt_tag_id,ptrt_tag_name  FROM `unittest_pagetriage_tags` (tests/phpunit/MediaWikiTestCase.php defined that table prefix)
The odds of that table _not_ containing the correct data are quite reasonable (I'm running into the problem), so $tags will be empty, causing $tags['user_name'] to throw a notice upon running tests.

Change-Id: If991f1a54723575acfac627a5759e386ccb67c58

wikimedia/mediawiki-extensions-PageTriage,wikimedia/mediawiki-extensions-PageTriage,wikimedia/mediawiki-extensions-PageTriage