Fixing Dataserver token handling

kuksa-client/kuksa_client/cli_backend/ws.py

@@ -297,8 +297,13 @@ async def connect(self, _=None):
         subprotocols = ["VISSv2"]
         if not self.insecure:
             context = ssl.create_default_context()
-            context.load_cert_chain(
-                certfile=self.certificate, keyfile=self.keyfile)
+            if self.certificate is not None and \
+               self.keyfile is not None:
+                logger.debug("Credentials used for mutual authentication; "
+                             "Client certificate file = %s, Client key file =  %s",
+                             self.certificate, self.keyfile)
+                context.load_cert_chain(
+                    certfile=self.certificate, keyfile=self.keyfile)
             context.load_verify_locations(cafile=self.cacertificate)
             # We want host name to match
             # For example certificates we use subjectAltName to make it match for Server, localahost and 127.0.0.1