* Empty date (initialized with Date::EMPTY == all 0) stays empty.
* Day 0 is normalized to the last day of the previous month.
* Day 0 AND month 0 is also normalized to the last day of the previous month
  (and not the last day of the previous-1 month).
* Before,
  * all day 0 cases weren't handled at all and day 0 stayed day 0, which
    doesn't make sense in a Normalize() context
  * empty date wasn't detected and resulted in nonsense date -0001-12-00

Change-Id: Ib12ff7694e1adb17724af038a266f6f284285f36