Skip consistency check when root is size zero

[hickford]

Any head is consistent with empty root.

Checklist

• I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• I have updated documentation accordingly.

[hickford]

Motivation: some logs reject get-sth-consistency requests with first=0.

> http get "https://yeti2022-2.ct.digicert.com/log/ct/v1/get-sth-consistency?first=0&second=1"
HTTP/1.1 400 BAD REQUEST

while other logs respond with an empty proof:

> http get "https://oak.ct.letsencrypt.org/2022/ct/v1/get-sth-consistency?first=0&second=1"
HTTP/1.1 200 OK
{
    "consistency": []
}

The RFC doesn't guarantee at STH at size 0 exists https://datatracker.ietf.org/doc/html/rfc6962#section-4.4

Both tree sizes must be from existing v1 STHs (Signed Tree Heads).