[GettingStarted] Ignore empty messages

Spotted by Toliño
https://translatewiki.net/wiki/Thread:Translating_talk:MediaWiki/About_MediaWiki:Gettingstarted-task-1/en,_MediaWiki:Gettingstarted-task-2/en_and_MediaWiki:Gettingstarted-task-3/en

Change-Id: Ib2c02fbf8826b02f985653f41123d6a95545af16