eventView.js: (minor) Fix possible undefined argument for

set_events().

The current selected date may not have any events, so set_events()
would receive 'undefined' for the first argument.

Observed in logs as:
... JS ERROR: TypeError: event_data_list is undefined
... set_events@eventView.js:757:13
... _perform_gc@eventView.js:410:30

... JS LOG: [LookingGlass/warning] Invalid or null source id used when attempting to run Mainloop.source_remove()
... global.dump_gjs_stack@overrides.js:24:42
... Mainloop.source_remove@overrides.js:167:20
... _stop_gc_timer@eventView.js:384:22
... _start_gc_timer@eventView.js:390:14
... _handle_added_or_updated_events@eventView.js:453:14

files/usr/share/cinnamon/applets/calendar@cinnamon.org/eventView.js

@@ -405,7 +405,7 @@ class EventsManager {
         }
 
         if (any_removed) {
-            this._event_list.set_events(this.events_by_date[this.current_selected_date.to_unix()]);
+            this._event_list.set_events(this.events_by_date[this.current_selected_date.to_unix()] || null);
             this.emit("events-updated");
         }
 
@@ -445,7 +445,7 @@ class EventsManager {
         }
 
         if (changed) {
-            this._event_list.set_events(this.events_by_date[this.current_selected_date.to_unix()]);
+            this._event_list.set_events(this.events_by_date[this.current_selected_date.to_unix()] || null);
         }
 
         this._start_gc_timer();