Fix bug where Milestone page led to console error

Fixes a bug where Group Milestone page led to console
error when Apollo Query is unavailable.

Changelog: fixed

app/assets/javascripts/sidebar/components/time_tracking/time_tracker.vue

@@ -108,7 +108,7 @@ export default {
   },
   computed: {
     isTimeTrackingInfoLoading() {
-      return this.$apollo?.queries.issuableTimeTracking.loading ?? false;
+      return this.$apollo?.queries.issuableTimeTracking?.loading ?? false;
     },
     timeEstimate() {
       return this.timeTracking?.timeEstimate || 0;