convoluted error handling

[komuw]

• Most implementations[1][2] of raft StableStore interface; when you do a get for a key and the value returned happens to be a nil []byte, then they return an error.
• Since we are using StableStore interface to persist (ballots and state), we need to check if the state exists(ie do a get). But instead of throwing an error if state is nil []byte, we need to 'catch' it and set it to nil.
• it looks hideous and should be replaced with something better.

ref:

1. https://github.com/hashicorp/raft-boltdb/blob/6e5ba93211eaf8d9a2ad7e41ffad8c6f160f9fe3/bolt_store.go#L243-L245
2. https://github.com/hashicorp/raft-mdb/blob/55f29473b9e604b3678b93a8433a6cf089e70f76/mdb_store.go#L328-L329