Type error when calling get_monitor() on a monitor with a null silence downtime
**Describe the bug**

I have a monitor that is returning `"silenced":{"*":null}` from the GET API call, i.e. it is referencing a null downtime.  This causes `get_monitor()` to fail unless I specify `_check_return_type=False`:

`datadog_api_client.v1.exceptions.ApiTypeError: Invalid type for variable '*'. Required value type is int and passed type was NoneType at ['received_data']['options']['silenced']['*']`

It looks like this should be easily fixed by updating the type expectation to include `none_type` (PR forthcoming)

**To Reproduce**

Have a silenced monitor without associated downtime and try to call `get_monitor()` on it.

**Expected behavior**

get_monitor() should not fail type checking

**Environment and Versions (please complete the following information):**

The bug is present in HEAD
diff --git a/src/datadog_api_client/v1/model/monitor_options.py b/src/datadog_api_client/v1/model/monitor_options.py
index 6ef9649bc..276a6238c 100644
--- a/src/datadog_api_client/v1/model/monitor_options.py
+++ b/src/datadog_api_client/v1/model/monitor_options.py
@@ -59,7 +59,7 @@ class MonitorOptions(ModelNormal):
             "renotify_occurrences": (int, none_type),
             "renotify_statuses": ([MonitorRenotifyStatusType], none_type),
             "require_full_window": (bool,),
-            "silenced": ({str: (int,)},),
+            "silenced": ({str: (int, none_type)},),
             "synthetics_check_id": (str, none_type),
             "threshold_windows": (MonitorThresholdWindowOptions,),
             "thresholds": (MonitorThresholds,),
['src/datadog_api_client/v1/model/monitor_options.py']