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.

src/oscar/views/generic.py

@@ -192,7 +192,7 @@ class PhoneNumberMixin(object):
 
         # empty
         if number in validators.EMPTY_VALUES:
-            return None
+            return ''
 
         # Check for an international phone format
         try: