Provide deeper guidance on 0 amount settings for Lightning invoices

[moneyball]

Attached is my own personal testing of wallets with 0 amounts. It is a total mess! Muun and Phoenix default to 0 amount, yet most wallets give an error when using them, including Muun!

Hopefully wallets converge on supporting this so this matrix isn’t a mess, but in the mean time the design guide could help a product builder out.

1. Should a wallet support 0 amount receiving? (I think yes)
2. Should a wallet default to 0 amount receiving? (I think it depends on use case…but also needs to take into account the current mess of wallet support)
3. Should a wallet support sending to 0 amount invoices (ie let sender enter amount)? (I think yes, obviously…only reason we’re in this mess is due to an LND security bug that has been fixed but wasn't broadly communicated so the ecosystem is unclear on this)

https://bitcoin.design/guide/payments/request/#amount

[Bosch-0]

Also tested on Blixt, you can generate and send 0 amount invoices fine

[Bosch-0]

Muun allows sending to zero amount invoices on android but not iOS. An issue already existed for this muun/falcon#15

Breez Issue existed from back in August for generating zero amount invoices, bumped the issue up for better visibility breez/breezmobile#556

[Bosch-0]

Interesting comment from Breez on this breez/breezmobile#556 (comment)

Receiving zero amount invoices requires changes to the on-the-fly channel creation infrastructure.

[Bosch-0]

From tbast at Phoenix: ACINQ/phoenix#248 (comment)

No, this warning cannot be removed yet.
We need recipients to implement lightning/bolts#829 to be completely safe.
Before that, our node, if malicious, could in theory steal the payment, so users should be made aware.