added catch for TypeError
added catch for TypeError

when value is not required and save empty string, form clean make it None value. But None value bad for strptime method of datetime
helber/django-dbsettings,sciyoshi/django-dbsettings,zlorf/django-dbsettings,johnpaulett/django-dbsettings,helber/django-dbsettings,DjangoAdminHackers/django-dbsettings,zlorf/django-dbsettings,sciyoshi/django-dbsettings,nwaxiomatic/django-dbsettings,winfieldco/django-dbsettings,nwaxiomatic/django-dbsettings,DjangoAdminHackers/django-dbsettings,johnpaulett/django-dbsettings,winfieldco/django-dbsettings