forms: Fix to_python method signature.
forms: Fix to_python method signature.

According to the documentation, `to_python` should account for `None`
being a possible value to be processed.

See also: https://docs.djangoproject.com/en/4.0/howto/custom-model-fields/#converting-values-to-python-objects.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip