Fix compatibility with django-phonenumbers-field 1.3.0
Return an empty string when no value was given for a phone number instead
of None since the phonenumberfield doesn't cast none to '' anymore which
resulted in an not null sql error.
@@ -192,7 +192,7 @@ class PhoneNumberMixin(object):
# empty
if number in validators.EMPTY_VALUES:
- return None
+ return ''
# Check for an international phone format
try:
