Date fields do not accept '__null__' cuts

[pablolmedorado]

I'm working with version 1.1 of Cubes.

When I try to filter a date type field (role: time), I can't use the 'null' value.

The error I get is 'TypeError: expected string or bytes-like object' (calendar.py line 252) https://github.com/DataBrewery/cubes/blob/f3364d9d4d77f676d451a68d8bd39b5dd36b3e8f/cubes/calendar.py#L252

Is there a way to filter by null date?

Related issue: https://github.com/DataBrewery/cubes/issues/437

Thanks in advance.