new/edit description should be optional

[allanwind]

(venv) allan@vent:~/src/khal$ khal new -i
summary: test
datetime range: 
datetime range: now
timezone [EST5EDT]: 
description (or "None"): 
description (or "None"):     

I.e. the first description should be accepted as valid input. I have the trivial fix ready to go once #1052 is merged.

Btw, if there is interest in #1050 then "accept default" and the "unset field" would become natural than the magic "None" string as either accepting the (possible) edited value and if new value is '' then it means unset.