Field.get_value should not raise exception if followed attribute returns None

Author: bmihelac

import_export/fields.py

@@ -64,6 +64,8 @@ def get_value(self, obj):
                         repr(value), attr)
                 raise exceptions.FieldError(msg)
             value = getattr(value, attr)
+            if value is None:
+                return None
 
         if callable(value):
             value = value()