Modify NewBlockTemplate to accept nil pay addr.

There are certain cases such as getblocktemplate which allow external
callers to be repsonsible for creating their own coinbase to replace the
generated one.  By allowing the pay address to be nil in such cases, the
need to specify mining addresses via --miningaddr can be avoided thereby
leaving the payment address management up to the caller.
Update for recent btcjson changes.

This commit updates the types to match the recent changes to the btcjson
result types.