CoinChooser: avoid NotEnoughFunds if zero buckets are sufficient

[JeremyRand]

The coin chooser incorrectly raises NotEnoughFunds when the following conditions are both true when make_tx is called:

• len(coins) == 0
• The combined value of inputs is sufficient to fund the transaction.

This PR explicitly checks whether an empty bucket is sufficient to fund the transaction, and if so, suppresses the NotEnoughFunds.

[SomberNight]

Thank you for the report and the code contribution.
I've adapted your change a bit; I think it's more natural to do the check in bucket_candidates_any.