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

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