Fix NPE bug in mongo plugin

[jaehong-kim]

Load of known null value.

        final ServerAddress serverAddress = ArrayArgumentUtils.getArgument(args, 0, ServerAddress.class);
        if (serverAddress != null) {
            final String hostAddress = HostAndPort.toHostAndPortString(serverAddress.getHost(), serverAddress.getPort());
            hostList.add(hostAddress);
        } else {
            // arg0 is List<ServerAddress>
            final List list = ArrayArgumentUtils.getArgument(args, 0, List.class);
            if (list != null) {
                for (Object o : list) {
                    if (o instanceof ServerAddress) {
                        final String hostAddress = HostAndPort.toHostAndPortString(serverAddress.getHost(), serverAddress.getPort());