Support case when no certificate is passed.

slapos/libnetworkcache.py

@@ -189,6 +189,7 @@ class NetworkcacheClient(object):
       Returns true if it can find any valid certificate or false if it does not
       find any.
     """
+    if self.signature_certificate_list is not None:
       for certificate in self.signature_certificate_list:
         if self._verifySignatureCertificate(signature_string, certificate):
           return True