Miner control functions should abort for empty params

[wadealexc]

• ChangePeerID should check that params.NewID is not empty

• ChangeMultiaddrs should check that params.NewMultiaddrs is not empty

[anorth]

I'm not sure that the chain should be enforcing this. They're both opaque values as far as the rest of the chain state is concerned. If a miner doesn't want to advertise one of them for some reason, do we want to force them to put junk in there?

@whyrusleeping do you have an opinion here?

[anorth]

I'm going to close this in favour of #804. FYI @Stebalien please consider any minimum lengths we should also be enforcing.