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
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


(cherry picked from commit 50b7739076)

js/ui/dateMenu.js

@@ -118,7 +118,7 @@ var WorldClocksSection = class WorldClocksSection {
             if (!clocks[i].location)
                 continue;
             let l = world.deserialize(clocks[i].location);
-            if (l)
+            if (l && l.get_timezone() != null)
                 this._locations.push({ location: l });
         }