Rename `payment_parameters_from_zero_amount_invoice` to imply amount-less, not 0-amount

[TheBlueMatt]

lightning-invoice treats missing amount differently from set amounts, but the whole zero-amount thing is about...zero amounts. Do we actually support zero amounts properly or only invoices with no amount (which presumably other lightning nodes can't create)? Should we be refusing to deserialize invoices with no amount at all?

[jkczyz]

Hmm... do you have more background on this issue? I always thought "zero-amount" was synonymous with "amount-less" but that the latter was the accurate term.

[jkczyz]

Here are old issues from other implementations for amount-less invoices:

• https://github.com/ElementsProject/lightning/issues/534
• https://github.com/lightningnetwork/lnd/issues/564
• https://github.com/ACINQ/eclair/issues/1478#issuecomment-652851241

[TheBlueMatt]

Oh, okay, nevermind. I managed to completely confuse myself based on the "0 amount" name. We should probably rename the references to it so that its less confusing for others.

[tnull]

FWIW, in LDK Node we use a "variable amount" naming pattern, but mention "zero-amount" in the docs as users might try to find the functionality by this (confusing) name.

[srikanth-iyengar]

I would like to work on this issue, is this up for grabs ?

[mubarak23]

@TheBlueMatt can i work on this