When paying a 0 amount invoice, after clicking pay the entered amount
will get saved in
`PaymentIdentifier.boltPaymentIdentifier.bolt11.amount_msat`. However if
the user clicks cancel instead of pay, and then changes the amount,
amount_msat will not get updated, as it is not None anymore. If the user
clicks pay again, it will pay the previously entered wrong amount.
This checks the `LnAddr` amount instead of the `Invoice` amount.
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