Review witness handling of empty log states

[mhutchinson]

The checkpoint for an empty log (size 0) is a known special case. Consistency proofs from this size don't make sense because the empty hash is not actually a prefix. In essence, anyone that takes a Checkpoint of size 0 must TOFU the next tree size they receive for this log.

transparency-dev/merkle#140 makes a change to make this more explicit. It's likely that when this witness library is updated to pull in that change that witnesses that see a log first at size 0 will fail to verify consistency, based on my reading of the code.

TODO:

• add tests for this case
• support ratcheting forward from tree size 0 checkpoints

Note that this really is an edge case as witnesses are very unlikely to see a log starting in the empty state. By the time logs are added to a witness network, they have always had contents, and this is likely to continue. That said, having a case where the witness can fail to progress for a log is not good and should be fixed.