datetime: fix invalid representation of timestamps with fraction part

Sometimes we need negative timestamps to work with dates before
1970. But seems such cases were even covered in tests. So there
wasn't any handling of negative timestamps with fraction part.
Such datetime objects had incorrect string representation (e.g.
"1963-11-22T12:30:02.-999"). This patch fixes it.

Closes #8570

NO_DOC=bugfix

(cherry picked from commit 8e7514b9)