Set context user as author/creator when the Confluence package does not contain any author/creator

Steps to reproduce:

1. Create a page in Confluence
2. Click on "Space Tools - Content Tools - Export - XML"
3. Go to your wiki
4. Install WikiStream
5. Import the page from Confluence

• Input type: Confluence XML input stream (confluence + xml)
• The Source
• Output type: XWiki instance output stream (xwiki + instance)

6. Click on the "Convert" button
7. See the creator of the page

On the page: "Created by: Unknown User"
On Activity Stream: "XWikiGuest"