Saveguard calendarSelectedFeedsMixin calendar-query against no data

This threw an error on mount previously.

(cherry picked from commit e13ac755)

aleksis/core/frontend/components/calendar/calendarSelectedFeedsMixin.js

@@ -17,9 +17,11 @@ const calendarSelectedFeedsMixin = {
     calendar: {
       query: gqlCalendarFeeds,
       result({ data }) {
-        this.selectedCalendarFeedNames = data.calendar.calendarFeeds
-          .filter((c) => c.activated)
-          .map((c) => c.name);
+        if (data) {
+          this.selectedCalendarFeedNames = data.calendar.calendarFeeds
+            .filter((c) => c.activated)
+            .map((c) => c.name);
+        }
       },
     },
   },