org: Set ? for unknown values in Agenda view.

* org-weather-metno.el: "?" as default value
* weather-metno-query.el: Added support for default values in `weather-metno-query-format'
* query-test.el: Updated and added test for
  `weather-metno-query-format'

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>