Apparently this problem is a symptom of Mail running in multitask mode. Here is a simple fix that should work for you until the problem is addressed by Apple.

Quit the Mail app from the multitask bar.

Here’s how to do it:

1. Click the Home button to exit the Mail app.
2. Double-click the Home button to bring up the multitask bar listing currently “open” apps.
3. Locate the Mail app icon in the multitask bar.
4. Tap and hold the icon until the red minus sign appears.
5. Tap the red minus.
6. The Mail icon will vanish. You have now completely quit the Mail app.
7. Double-click the Home button to exit the multitask bar.

When you next launch Mail the messages with no sender and no content should be gone.