no way to use empty strings to create eg `wait.none:` filter

[smemsh]

use of empty string, eg passing to filter method like: .filter(wait__any='') will fail:

(Pdb) p normalize_func
<bound method SerializingObject.normalize_wait of
<tasklib.filters.TaskWarriorFilter object at 0x7f2428eaf250>>
(Pdb) p normalize_func('')
*** tasklib.backends.TaskWarriorException: No expression to evaluate.

this surely affects other things than wait because each one is normalized and deserialized differently. but wait is the one I want to use it for at the moment...