fixup: toy example previously could loop forever

This is a toy example where, presumably, `username` had a NOT NULL constraint as the real `email` column does, but this makes it explicit that these are concerns for these kinds of migrations.

Author: MGibson1

docs/contributing/database-migrations/edd.mdx

@@ -226,7 +226,7 @@ WHILE @RowsAffected > 0
 BEGIN
     UPDATE TOP (@BatchSize) [dbo].[User]
     SET email = username
-    WHERE email IS NULL;
+    WHERE email IS NULL AND username IS NOT NULL;
 
     SET @RowsAffected = @@ROWCOUNT;
 END