There is a suggestion to set up a rule to move them to junk (personally I don't like this, I shouldn't have to set up rules to deal with spam/system errors). This Apple thread also seems to confirm that it's MobileMe related.
A friend of mine contacted Apple recently and was told that the messages don't begin as blank. They are actually spam that has had their data filtered out before they arrive to your inbox. He said the messages should be harmless and that you should go ahead & delete them. Apple is working on a fix that will prevent them from arriving to our inboxes.
There is a suggestion to set up a rule to move them to junk (personally I don't like this, I shouldn't have to set up rules to deal with spam/system errors). This Apple thread also seems to confirm that it's MobileMe related.