blockchain: Handle genesis auto revocation agenda.

[rstaudt2]

This updates the IsAutoRevocationsAgendaActive function to return false for the genesis block.

[chappjc]

Before

$ dcrctl getblock 298e5cc3d985bfe7f81dc135f360abe089edd4396b86d2de66b0cef42b21d980 true
-32603: hash 0000000000000000000000000000000000000000000000000000000000000000 does not exist

After

$ dcrctl getblock 298e5cc3d985bfe7f81dc135f360abe089edd4396b86d2de66b0cef42b21d980 true
{
  "hash": "298e5cc3d985bfe7f81dc135f360abe089edd4396b86d2de66b0cef42b21d980",
  "confirmations": 595618,
  "size": 0,
  "height": 0,
  "version": 1,
  "merkleroot": "66aa7491b9adce110585ccab7e3fb5fe280de174530cca10eba2c6c3df01c10d",
  "stakeroot": "0000000000000000000000000000000000000000000000000000000000000000",
  "tx": [
    "e7dfbceac9fccd6025c70a1dfa9302b3e7b5aa22fa51c98a69164ad403d60a2c"
  ],
  "time": 1454954400,
  "mediantime": 1454954400,
  "nonce": 0,
  "votebits": 0,
  "finalstate": "000000000000",
  "voters": 0,
  "freshstake": 0,
  "revocations": 0,
  "poolsize": 0,
  "bits": "1b01ffff",
  "sbits": 2,
  "extradata": "0000000000000000000000000000000000000000000000000000000000000000",
  "stakeversion": 0,
  "difficulty": 32767.74999809,
  "chainwork": "0000000000000000000000000000000000000000000000000000800040002000",
  "previousblockhash": "0000000000000000000000000000000000000000000000000000000000000000",
  "nextblockhash": "000000000000437482b6d47f82f374cde539440ddb108b0a76886f0d87d126b9"
}