E3S7A.1, E3S8A: Prevent SIGSEGV on Wesnoth 1.11.8 due to missing ToD schedule

A change in Wesnoth 1.11.8 causes the game to not default-construct a
ToD object in case the scenario lacks a ToD schedule. Therefore, we must
default-construct our own using an empty [time] tag if we don't want to
crash badly on prestart.