Do not add None to samples in case of connection error

Otherwise we get `TypeError: unsupported operand type(s) for +: 'float'
and 'NoneType'` when calculating the average.

scripts-available/CDB_FederatedServerDiagnostics.sql

@@ -151,6 +151,7 @@ AS $$
 
     t_start = timer()
     samples = []
+    n_errors = 0
 
     for i in xrange(n_samples):
         try:
@@ -159,13 +160,13 @@ AS $$
             s.shutdown(socket.SHUT_RD)
         except (socket.timeout, OSError, socket.error), ex:
             plpy.warning('could not connect to server %s:%d, %s' % (host, port, str(ex)))
-            samples.append(None)
+            n_errors += 1
 
         t_connect = (t_stop - t_start) * 1000.0
         plpy.debug('TCP connection %s:%d time=%.2f ms' % (host, port, t_connect))
         samples.append(t_connect)
 
-    return sum(samples) / n_samples
+    return sum(samples) / len(samples)
 $$
 LANGUAGE plpythonu VOLATILE PARALLEL UNSAFE;