handle only non-empty usernames in CommaSeparatedUserField.clean. Thanks for the fix to  novotny.radim. Fixes issue #51.
handle only non-empty usernames in CommaSeparatedUserField.clean. Thanks for the fix to  novotny.radim. Fixes issue #51.

JordanReiter/django-messages,arneb/django-messages,gustavoam/django-messages,nikhil-above/django-messages,HiddenClever/django-messages,HiddenClever/django-messages,nikhil-above/django-messages,brajeshvit/Messagemodule,JordanReiter/django-messages,arneb/django-messages,cca/django-messages,procrasti/django-messages,fpischedda/django-messages,procrasti/django-messages,Chris7/django-messages,gustavoam/django-messages,tobiasgoecke/django-messages,Chris7/django-messages,tobiasgoecke/django-messages,lupinogle/django-messages,brajeshvit/Messagemodule,fpischedda/django-messages,andyzsf/django-messages,andyzsf/django-messages,michaelmior/django-messages,Bunch/django-messages