Fixes #2143 - PUTs to Site Endpoint Requires Value for time_zone

Allow null values for `time_zone` field in the writeable serializer for the sites endpoint.

netbox/dcim/api/serializers.py

@@ -80,7 +80,7 @@ class NestedSiteSerializer(serializers.ModelSerializer):
 
 
 class WritableSiteSerializer(CustomFieldModelSerializer):
-    time_zone = TimeZoneField(required=False)
+    time_zone = TimeZoneField(required=False, allow_null=True)
 
     class Meta:
         model = Site