worldClocks: Ignore locations with unknown timezone

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 d7682676ac9 moved weather stations from cities to countries - the station itself is unlikely 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.

Fixes #1062 (closed)