monitoring/zabbix_host: Fix (no) proxy handling

When updating a host with no proxy explicitly set,
the host was always reported as changed, because it was
comparing `"0"` and `None`.

monitoring/zabbix_host.py

@@ -362,6 +362,7 @@ class Host(object):
         if set(list(template_ids)) != set(exist_template_ids):
             return True
 
+        if proxy_id is not None:
             if host['proxy_hostid'] != proxy_id:
                 return True