[datadog_downtimes_schedule] Fix downtimes schedule with monitor id identifier

[skarimo]

We need to set the types.Set explicitly to null other wise it will default to model type which omits the element type. I will put up a follow up pr for more permanent fix.

E.g.:

type MonitorIdentifierModel struct {
	DowntimeMonitorIdentifierId   types.Int64 `tfsdk:"monitor_id"`
	DowntimeMonitorIdentifierTags types.Set   `tfsdk:"monitor_tags"`
}

Expected type in schema:

types.Set<types.StringType>