Bug: Handle Receive case where capacity is 0, and amount is 0

[danielnordh]

If wallet has 0 channels / 0 receive capacity, a regular bolt11/bolt12 is still generated.

Investigate if a 0 amount JIT invoice is possible, if so use it instead of regular.
If not, maybe we should remove the regular bolt11 as it will always fail.

[danielnordh]

0 amount JIT is not possible.

We currently check:

let needsJIT = amountSat.satsAsMsats > maxReceiveCapacity()

Both outside, and inside generateUnifiedWithJIT()
So a fix needs to change this, and could maybe simplify to only check in one place (but could be good fallback)