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

@@ -362,8 +362,9 @@ class Host(object):
         if set(list(template_ids)) != set(exist_template_ids):
             return True
 
-        if host['proxy_hostid'] != proxy_id:
-            return True
+        if proxy_id is not None:
+            if host['proxy_hostid'] != proxy_id:
+                return True
 
         return False