We currently assume that every location has an associated timezone.
While this is sound in the real world, in practise it depends on
whether or not libgweather can find a corresponding timezone DB
entry.
This used to be a fringe case, but has become more likely when commit
https://gitlab.gnome.org/GNOME/libgweather/commit/d7682676ac9 moved
weather stations from cities to countries - the station itself is un-
likely to have a timezone entry, and the country may be part of more
than a single timezone.
It would be good for libgweather to return a timezone for those
locations again, but we should defend against the case anyway.
We cannot tell what time it is at a particular location without
knowing the timezone, so simply filter them out.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1062
50b7739076