nwc: pay_invoice honors request amount for amountless bolt11

pay_invoice derived the amount solely from the decoded invoice, so an
amountless bolt11 yielded amount = round(undefined/1000) = NaN and
sendLightning rejected it with "Invalid amount". NIP-47 allows the client
to specify the amount (msat) in the request for exactly this case (AlbyHub
supports it; Alby Go and others hit the error). Fall back to the request's
`amount` field when the invoice carries none; invoice amount still takes
precedence when present, and a still-missing amount returns a clear error
instead of NaN. sendLightning/sendInternal already handle amountless
invoices given a valid amount — the bug was only the handler dropping it.

Fixes #83

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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