fix lnbits create_invoice error response

[aki-mizu]

In latest version of lnbits, "/api/v1/payments" endpoint does not allow null value in create_invoice optional request parameter, this pull request fixes the issue so lnbits can be used for mint backend. I verified it works for Alby as well, but I don't have Strike account so I cannot verify if it works on Strike or not. Hopefully someone can test it on Strike as well.