Support case when no certificate is passed.

slapos/libnetworkcache.py

@@ -189,9 +189,10 @@ class NetworkcacheClient(object):
       Returns true if it can find any valid certificate or false if it does not
       find any.
     """
-    for certificate in self.signature_certificate_list:
-      if self._verifySignatureCertificate(signature_string, certificate):
-        return True
+    if self.signature_certificate_list is not None:
+      for certificate in self.signature_certificate_list:
+        if self._verifySignatureCertificate(signature_string, certificate):
+          return True
     return False
 
   def _verifySignatureCertificate(self, signature_string, certificate):