Special-case the 0th minimum bucket in historical scoring

Special-case the 0th minimum bucket in historical scoring

Points in the 0th minimum bucket either indicate we sent a payment
which is < 1/16,384th of the channel's capacity or, more likely,
we failed to send a payment. In either case, averaging the success
probability across the full range of upper-bounds doesn't make a
whole lot of sense - if we've never managed to send a "real"
payment over a channel, we should be considering it quite poor.

To address this, we special-case the 0th minimum bucket and only
look at the largest-offset max bucket when calculating the success
probability.