Handle timenode condition properly. Begin and End of
TimenodeContainer allow Sequence, Event, Timing, double,
and VOID. Only events with Source set are valid.

Change-Id: I25519110715072ca36b7f5c55c26415f9fbe9deb
Reviewed-on: https://gerrit.libreoffice.org/68210
Tested-by: Jenkins
Reviewed-by: Mark Hung <marklh9@gmail.com>