Empty lines are removed in enter mode BR

[Jakub Ś]

To reproduce:

1. Set enter mode to BR
2. Switch to source and paste below code or in WYSIWYG press Enter key few times (you will get the same)

   <br />
   <br />
   <br />
   &nbsp;
   

3. Toggle between WYSIWYG and Source

Result: Every time you switch one BR gets removed until there is nothing in editor (in last step   gets removed as well).

Problem can be reproduced in all browsers from CKEditor 4 beta. It doesn't occur in 3.x branch.