[POS] manage NULL error on BTC Invoice

[jambtc]

after invoking invoice, the server response is null

Then the function catch an error: data.error is wrong.

 $.ajax({
            url:'/index.php?r=invoices/Btcpayserver',
            type: "POST",
            beforeSend: function() {
                $('#waiting_span-btc').hide();
                $('#waiting_span-btc').after('<div class="waiting_span"><center><img width=25 src="'+ajax_loader_url+'"></center></div>');
            },
            data:{
                'id_pos'		: '4',
                'amount'		: amount_val,
            },
            dataType: "json",
            success:function(data){
                $('.waiting_span').remove();
                $('#waiting_span-btc').show();
                console.log(data);

                if (data.error){
                    $('.error-header').addClass("bg-danger").show();
                    $('.error-message').text(data.error);
                    return false;
                }else{
                    if ('Btcpayserver' != 'Bitpay')
                        window.location.href = data.url;
                    else
                        top.location = data.url;
                }
            },