A blank submission is created for file upload submission types when a teacher sets a students submission back to "Allow another attempt".  This then can count towards the students total submissions and can cause confusion to students and teachers.  I can see why this may be useful for online text submissions where the text of the original submission is copied over to this blank submission when they click on the "Add a new attempt based on previous submission" but this should happen on that event rather than by the teacher.

Steps to reproduce:
Create a new File upload assignment which allows for 3 attempts
Log in as a student and submit a file
Grade this and set the "Allow another attempt" to yes (or set it to No and then later on the view all submissions page click edit -> allow another attempt
The result is a blank second attempt.
 
If the student resubmits using the button "Add a new attempt based on previous submission" then this replaces the blank second attempt.
If the student resubmits using the button "Add a new attempt" then this creates a third attempt which can be viewed.