web: make 'log_access' robust to when host is None

Sometimes we get these error:

(process:28298): libsoup-WARNING **: soup-socket.c:1361: could not get remote address: Bad file descriptor
(process:28298): libsoup-CRITICAL **: soup_address_get_physical: assertion `SOUP_IS_ADDRESS (addr)' failed

When this happens the 'host' parameter passed to the
'WebServer.log_access' method will be None.