time-zone-monitor: Use local timezone as fallback without dbus service

On systems without anything providing the org.freedesktop.timedate1
interface, the timezone returned by the monitor would always be NULL,
because it was never set. This can lead to crashes in places that expect
the monitor to always return a valid timezone. So in that case use the
local timezone from when the monitor got initialized.

Fixes GNOME/gnome-calendar#703