TimeZoneSerializerField does not handle the empty string case

[sangiovannidev]

Hi, thanks for your hard work on django-timezone-field!

TimeZoneSerializerField.to_internal_value() seems not to fully handle the empty string ("") case.
With use_pytz=False, when the empty string is fed to the the method, it is passed to ZoneInfo(), which raises a ValueError, as follows:

>>> from zoneinfo import ZoneInfo
>>> ZoneInfo('')
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/zoneinfo/_tzpath.py", line 67, in find_tzfile
    _validate_tzfile_path(key)
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/zoneinfo/_tzpath.py", line 91, in _validate_tzfile_path
    raise ValueError(
ValueError: ZoneInfo keys must be normalized relative paths, got:

I would expect to_internal_value() to raise a ValidationError instead, which could be handled appropriately by DRF's exception handling machinery.

Please let me know if I can be of further help.

Thanks!

[ejain]

Shouldn't empty strings be allowed if the field has required=False and allow_null=True?

[mfogel]

@ejain after #137 is merged, empty strings will be allowed if allow_blank=True is specified as documented by DJRF for their CharField