self->thread can be NULL if gcal_calendar_monitor_set_range() was never
called prior to finalize. This is fine, so just check for NULL here
using g_clear_pointer().
Fixes:
[43/52] Compiling C object 'src/25a6634@@gcal-private@sta/core_gcal-calendar-monitor.c.o'
../../../../Projects/gnome-calendar/src/core/gcal-calendar-monitor.c: In function ‘on_client_view_objects_added_cb’:
../../../../Projects/gnome-calendar/src/core/gcal-calendar-monitor.c:402:16: warning: variable ‘old_size’ set but not used [-Wunused-but-set-variable]
402 | gint old_size;
| ^~~~~~~~