GtkCalendar can have an invalid date — mostly at initialization. This
means that GDateTime construction may fail. We need to handle that case
gracefully, like the old code did.

This fixes the `notify` test suite, which started failing with:

/Notification/GtkCalendar:
GLib-CRITICAL **: g_date_time_get_day_of_week: assertion 'datetime != NULL' failed

inside the Continuous builder.