Don't crash when adding invalid both dynafield and scannerfield

When adding both an invalid dynafield and an invalid scannerfield in the
same form, both fields come back as None, and then crashes. If added one
by one the correct error was shown, but of course this should work in
both cases.

postgresqleu/confreg/backendforms.py

@@ -125,8 +125,8 @@ def clean(self):
         if cleaned_data.get('checkinactive') and not cleaned_data.get('tickets'):
             self.add_error('checkinactive', 'Check-in cannot be activated if tickets are not used!')
 
-        dynafields = set([x for x in cleaned_data.get('dynafields').split(',') if x])
-        scannerfields = set([x for x in cleaned_data.get('scannerfields').split(',') if x])
+        dynafields = set([x for x in cleaned_data.get('dynafields', '').split(',') if x])
+        scannerfields = set([x for x in cleaned_data.get('scannerfields', '').split(',') if x])
         if scannerfields - dynafields:
             self.add_error('scannerfields', 'Only fields from the list of dynamic properties can be used. Incorrect fields: {}'.format(", ".join(scannerfields - dynafields)))