fix crash when trying to record before calibration

[bpinsard]

fix crash when trying to record before calibration when no past calibration data exists
If I understand correctly what that portion of code is doing, it should ignore when no calibration data are available, but it can raise another error than FileNotFoundError

example traceback before the fix

Process world:
Traceback (most recent call last):
  File "/home/basile/data/src/pupil/pupil_src/shared_modules/gaze_mapping/notifications.py", line 48, in from_dict
    dict_ = cls.sanitize_serialized_dict(dict_)
  File "/home/basile/data/src/pupil/pupil_src/shared_modules/gaze_mapping/notifications.py", line 107, in sanitize_serialized_dict
    return super().sanitize_serialized_dict(dict_)
  File "/home/basile/data/src/pupil/pupil_src/shared_modules/gaze_mapping/notifications.py", line 100, in sanitize_serialized_dict
    return super().sanitize_serialized_dict(dict_)
  File "/home/basile/data/src/pupil/pupil_src/shared_modules/gaze_mapping/notifications.py", line 58, in sanitize_serialized_dict
    dict_[field_name] = field_cls(dict_[field_name])
TypeError: 'ForwardRef' object is not callable