buildings-start_date: show 'unknown' when no start_date set

buildings-start_date.json

@@ -45,11 +45,15 @@
         "description": [
             "{% if tags.start_date %}",
             "{{ tags.start_date|osmFormatDate({ format: 'short' }) }}",
+            "{% else %}",
+            "{{ trans('unknown') }}",
             "{% endif %}"
         ],
         "popupDescription": [
             "{% if tags.start_date %}",
             "{{ tags.start_date|osmFormatDate }}",
+            "{% else %}",
+            "{{ trans('unknown') }}",
             "{% endif %}"
         ],
         "markerSymbol": null,