Fix 500 when saving /site/edit without email
When saving /site/edit with no no_reply_email set, a 500 error page
would be returned due to an empty value being passed as value to
the datastore (which is not allowed for email fields).
from django.dispatch import Signal
class_prepared = Signal(providing_args=["class"])
pre_init = Signal(providing_args=["instance", "args", "kwargs"])
post_init = Signal(providing_args=["instance"])
pre_save = Signal(providing_args=["instance", "raw"])
post_save = Signal(providing_args=["instance", "raw", "created"])
pre_delete = Signal(providing_args=["instance"])
post_delete = Signal(providing_args=["instance"])
post_syncdb = Signal(providing_args=["class", "app", "created_models", "verbosity", "interactive"])