What follows was observed using the Lighting Labs LND , but i think it happens with the LTC fork of it as well.
I am creating 0-amount HODL invoices via REST API https://localhost:8080/v2/invoices/hodl, like this one:
{
"value":0,
"expiry":31536000,
"r_preimage":"cFJMaBpeey0wu+tnbWD\/Wy0fbnQ4XXJvOnpfNDtDSDs=",
"hash":"FtMaCDxhyM4d7X4CbfilDQi+gcOH+usZ3Bpbn4fIqx4=",
"description_hash":"HOv7xHJJStMY\/cDIcX04UjTOju81t5BV3rD9+v2Hqe0="
}
This is the output payment request:
lntb1p0456rypp5zmf35zpuv8yvu80d0cpxm799p5ytaqwrslawkxwurfdelp7g4v0qhp5rn4lh3rjf99dxx8acry8zlfc2g6varh0xkmeq4w7kr7l4lv848kscqzpgxq97zvuqsp54y3ttl7gnu8a5j0dn572zxs5yve2zhvd7g03pqpk8kd03nwt4qyq9qy9qsqeuqef37m2w065rfuxxymcwukkzx8dhu40ezjrwsrpqku7jdl22e98v8skqvmvngsjjawawk672q2kceakqymhpwvea4pmtcym4vka0cq64a8nm
lncli listinvoices shows r_preimage = null for invoices like that.
Is that ok?
Did i do everything correctly while creating the invoice?
p.s.
I know i should not be using these type of invoices which are vulnerable according to Zero Amount Invoices - Suredbits, but in my case there is nothing to worry about, because i know in advance what amount the user is about to send me ( the user will pay via a LNURL-pay link, so the wallet will tell me the amount entered by the user via HTTP just before paying the invoice, thus i will know if i have to settle or cancel the HODL invoice based on the amount received).