Description of problem:
The Create User spoke doesn't allow to remove all characters from the "Username" field if the "Full name" field is not empty.

Version-Release number of selected component (if applicable):
Fedora-19-Alpha-TC1
anaconda-19.12

How reproducible:
always

Steps to Reproduce:
1. start installation in graphical mode
2. configure partitioning/package set and begin the installation
3. click on "CREATE USER"
4. enter your name to the "Full Name" field (entered "Jan Stodola")
5. switch to the "Username" field and try to delete pre-filled username (jstodola)
  
Actual results:
It is not possible to delete all characters from the "Username" field, as soon as all characters are deleted, anaconda resets the field to previously pre-filled username ("jstodola" in my case).

Expected results:
It is possible to delete all characters in the "Username" field.