fix default value when voting on eth1data

specs/validator/0_beacon-chain-validator.md

@@ -283,16 +283,20 @@ def is_candidate_block(block: Eth1Block, period_start: uint64) -> bool:
 ```python
 def get_eth1_vote(state: BeaconState, eth1_chain: Sequence[Eth1Block]) -> Eth1Data:
     period_start = voting_period_start_time(state)
-    # `eth1_chain` abstractly represents all blocks in the eth1 chain.
+    # `eth1_chain` abstractly represents all blocks in the eth1 chain sorted by descending block height
     votes_to_consider = [get_eth1_data(block) for block in eth1_chain if
                          is_candidate_block(block, period_start)]
 
+    # Valid votes already cast during this period
     valid_votes = [vote for vote in state.eth1_data_votes if vote in votes_to_consider]
 
+    # Default vote to use if no valid votes already cast in this period
+    default_vote = votes_to_consider[0] if any(votes_to_consider) else state.eth1_data
+
     return max(
         valid_votes,
         key=lambda v: (valid_votes.count(v), -valid_votes.index(v)),  # Tiebreak by smallest distance
-        default=get_eth1_data(ETH1_FOLLOW_DISTANCE),
+        default=default_vote
     )
 ```