Fixed: CPDateFormatter getObjectValue: with nil errorDescription crash.

Like for all formatters, a nil errorDescription just means we don't care about the error description if there is an error.

Fixes #2030.
Fixed: CPDateFormatter stringFromDate:"" did not return a default date.

In Cocoa formatting an empty string actually results in a particular date. For consistency we should handle an empty string the same way.

Refs #2030.