allow insights_system_id to be set to null via api

awx/api/serializers.py

@@ -1174,6 +1174,7 @@ class InventoryScriptSerializer(InventorySerializer):
 
 class HostSerializer(BaseSerializerWithVariables):
     show_capabilities = ['edit', 'delete']
+    insights_system_id = serializers.CharField(allow_blank=True, allow_null=True, required=False, default=None)
 
     class Meta:
         model = Host