Fix `RecurrenceRuleField.compress()` method for blank values.
Fix `RecurrenceRuleField.compress()` method for blank values.

ic-labs/django-icekit,ic-labs/icekit-events,ic-labs/django-icekit,ic-labs/django-icekit,ic-labs/icekit-events,ic-labs/icekit-events,ic-labs/django-icekit