Fixed #10121: Restored code lost in r9766 that prevented overwriting an...

Fixed #10121: Restored code lost in r9766 that prevented overwriting an already-set blank=True FileField with blank.  This would happen, for instance, in the admin if an object with a FileField was edited/saved but the file not re-uploaded, resulting in the association to the previously-uploaded file being lost.  Adding the ability to re-blank FileFields when they are once set is the subject of a different ticket (#7048), for now the pre-9766 behavior here has just been restored in order to avoid unexpected data loss.  Thanks to Alex for help in understanding how to fix this without un-doing the intent of r9766.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9840 bcc190cf-cafb-0310-a4f2-bffc1f526a37