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)

cubes/cubes/calendar.py

Line 252 in f3364d9

relative_match = RELATIVE_FINE_TIME_RX.match(reference)

Is there a way to filter by null date?

Related issue: #437

Thanks in advance.