[IMP] resource_booking: No create/unlink call if nothing to do

Depending on how the create/unlink overrides are programmed, there can
be overheads or blocking things when calling unconditionally these
methods, even with an empty list, so let's only call them when there
is really something to delete or to create.

This has been discovered as `google_calendar` module is calling Google
API on calendar event create (although vals_list being empty), and due
to an error with Google API right now, I was not able to create a
draft resource booking.

resource_booking/models/resource_booking.py

@@ -417,8 +417,10 @@ def _sync_meeting(self):
                     to_create.append(meeting_vals)
             else:
                 to_delete |= one.meeting_id
-        to_delete.unlink()
-        _self.env["calendar.event"].create(to_create)
+        if to_delete:
+            to_delete.unlink()
+        if to_create:
+            _self.env["calendar.event"].create(to_create)
 
     @api.constrains("combination_id", "meeting_id", "type_id")
     def _check_scheduling(self):