clock: fix handling of detached locations

GWeatherLocationEntry sometimes returns locations with level
GWEATHER_LOCATION_DETACHED. For such locations, weather_code is NULL
and clock_location_new crashes if we pass such code to it.

I can reproduce this by searching for "Riga" and selecting the first
entry from suggestions.
1 job for gweather-fixes in 7 minutes and 44 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #530026
build-ubuntu

00:07:44