[execo_g5k] get_host_longname: correct behaviour when site unknown

src/execo_g5k/api_utils.py

@@ -653,6 +653,8 @@ def get_host_longname(host):
         host_site = mo.group(3)
     else:
         host_site = get_host_site(host_shortname)
+        if not host_site:
+            return None
     return host_shortname + "." + host_site + ".grid5000.fr"
 
 def __get_site_metrics(site, grouped_hosts, metric, from_ts, to_ts, resolution):