Ed Koo
You could display all with two searches:
1. All messages from (none) by performing a search for Sender search set to "<>" (without quotes)
2. All messages that have a sender email address associated to them, by performing a search for Sender search set to "@" (without quotes).
I do not believe the two can be combined, will try as soon as I log in however and post the result.
Cheers.