[BUG] LNDHub extension: LNURL invoices can't be paid with Phoenix wallet

[petzsch]

Follow-up issue as requested here.

Steps to repro:

I have LNbits/LNDHub extension configured as backend for BTCPay Server (as custom lightning wallet). If I create a invoice with missing amount (BTCPay calls it a top-up invoice). I'm presented with a LNURL instead of a bolt11 invoice. This however can not be paid with Phoenix wallet, see the below screenshot (sorry for the german locale):

[tlindi]

I have encounted the same error with BTCPay Server 0.13.0 using LNbits LNDhub as backend - boltcard is not working.

Issue with loads of logs at BTCpay: btcpayserver/btcpayserver#5927