Do not try to verify empty signature.

slapos/libnetworkcache.py

@@ -213,6 +213,7 @@ class NetworkcacheClient(object):
     filtered_data_list = []
     if self.signature_certificate_list is not None:
       for data in data_list:
+        if len(data[1]):
           if self._verifySignatureInCertificateList(data[0], data[1]):
             filtered_data_list.append(data)
     else: