widgets: add a message where there's no available timetable (#23212)

wcs/qommon/templates/qommon/forms/widgets/select-timetable.html

@@ -103,6 +103,9 @@
         } else if (current_offset === null) {
           current_offset = 0;
         }
+        if (data.items.length == 0) {
+          $('<p class="timetable-widget--nothing">{% trans "No available timeslots." %}</p>').appendTo($table);
+        }
         var go_prev = $('<button class="prev" title="{% trans "Previous week" %}">←</button>');
         var go_next = $('<button class="next" title="{% trans "Next week" %}">→</button>');
         go_prev.prependTo($table);