Set genesis block body root to ssz hash of the empty struct rather than empty hash

[sorpaas]

A block is not useful unless it has a way to point to its parent block, so that an importer knows how to import it. Although the spec does not define it, all block intrinsically has a block identifier. Currently our block identifier is defined by previous_block_root:

signing_root(BeaconBlockHeader {
  	slot: block.slot,
	previous_block_root: block.previous_block_root,
	state_root: self.0.state_root,
	block_body_root: if is_genesis {
		Default::default()
	} else {
		hash(&block.body)
	},
	..Default::default()
})

Notice the if statement just for genesis block. If, instead, we change genesis block_body_root to empty struct signing root, it can be avoided.