Sharing content using Message Stream allows empty fields for users and groups

Sharing content using Message Stream allows empty fields for users and groups

When you want to share something wish a specific user or a specific group, leaving the field empty will still work.
This might render confusion because the user doesn't know the default behavior. Maybe we should make the field mandatory.

This issue depends on other issues, because we still have to specify the "XWiki." prefix, nor don't have an auto-complete feature yet, making it a not so easy to fix.