Make it possible to use the UserAvatar Macro without passing a fully quantified username

steps to reproduce:

• login
• on the main page, add the User Avatar and type your current username
• Save t he page, and you will get a velocity error
  "org.xwiki.rendering.macro.MacroExecutionException: User [Main.sorinie82] is not registered in this wiki"

If you go to "my Dashboard" and insert it from there, it will work from that page, but when enabling My Dashboard to be displayed as Main.WebHome, it doesn't work