DateTimeKey is capable of completing dates given with less than 3 parts (year,
month and day). This means less than 2 separators, not 3.
Also, be resilient to string which, when split by separators, generate an empty
list.
Add test cases.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@44048 20353a03-c40f-0410-a6d1-a30d3c3de9de